There's a TON of content in the Gimmiko demo. This guide can hopefully help you find some of it and stay alive long enough to get to it. Special thanks to the SorryPoniko! Discord for help with this guide, especially the excellent Porkyspal and NotCon.
Coinelius is the little Clippy wannabe on the edit screen. He'll give you a ton of super helpful tips not found in the tutorial and even let you play a game of solitaire. I'll be honest, I wrote some of my tips before realizing he already goes over them, and a lot more succinctly, too. Oh well, I guess now you just get the G-Zone spin on them as well.
If you find yourself taking damage too often, then you may have a case of not-jumping-enough-itis. You're totally invincible while airborne, and you can even bounce off enemies' heads to stay in the air longer. Some smaller enemies are even beaten when you jump on them. Translation: JUMP. Don't be afraid of the sky. You can bounce on enemies the microsecond you start descending, so you don't even have to be precise with it. I tend to just mash the button and I'm mostly alright. Dice surrounded by enemies? Jump in, grab it and jump out, real fast-like. Enemy spamming you with projectiles? Jump over them. The only downside to jumping is that your legs will get tired if you bounce on enemies too many times in a row, and you'll be forced to make an emergency landing.
If you're anything like me, you'll have had the idea to throw your dice at the wall to spam gimmicks. However, if you do that too much, your dice will start to glow red and quickly burn out. When it's burnt out, it can still be thrown, but gimmicks won't activate for a little bit. If you notice it starting to glow, you'd better let it cool off, as that takes significantly less time than waiting for it to reactivate after it burns out. You can still carry it around, just don't throw it for a few seconds. Oh yeah, and landing on a blank face cools it down a lot, too. Thanks, Coinelius!
If you jump around and get real close to enemies a lot like I do, it's important to understand squeeze frames. You won't take damage immediately upon touching an enemy; you'll start to turn red and then take damage when your squeeze frames run out. Basically, you have a little extra time to get away before losing a heart. You can even get more squeeze frames by buying Grazed Cheese from the One-Eight-One.
During a wave, there may be a few helpful things laying around for you to take advantage of. Most notably, there are occasionally these little bear-like creatures which don't count as enemies, but if you bounce on them, they'll curl up into a ball which you can throw as a weapon. You'll sometimes see gashapon machines which will dispense capsules containing what I call "minor summons", AKA summons with no lifebar, like cats and candy critters. There are also often small ceramic dice and vases that may spawn a single retsu each. You can break them by simply stepping on them. Better yet, you can find large brown pots which may contain a few retsu each. If you see a tree containing a beehive, throwing your object at it will summon bees to attack foes. A stage may contain shaking boxes with eyes peeking out. Break them to release extra enemies. I think that's everything...
You can tell how rare a gimmick is (and usually how good it is) depending on what color it's outlined in. Green is common, blue is uncommon, pink is rare and yellow is super rare. Rare doesn't always mean good, though —I'll gladly take Short Fuse over Mechanical Carp any day. Speaking of which, read on for more info on Mechanical Carp. It's also important to note that the more rare a gimmick is, the faster it'll overheat the object it's attached to.
On the Edit screen, pull up an object. To the right of its stats, you'll see two little icons: a green check and a peeling sticker. Clicking the check mark will disable and enable the selected object, letting you choose whether or not it will appear in the coming wave. Clicking the peeling sticker will pull up the sticker sheet, which allows you to select a sticker to put on that object. Stickers can help you differentiate between otherwise visually identical objects at a glance.
As Sprue will tell you, the dice/objects you can buy from her store, Sterling Customs, have a code which can indicate the objects' properties. However, it's not exactly straightforward. As of right now, the only aspect of the code I've really managed to crack is the most obvious—that the number at the end of the dice's name is the number of sides it has. I'm going to keep working on cracking the code, so expect updates!
Not counting the Tutorial character, Monaka, there are three characters in the demo, and two of them have to be unlocked. To unlock them, simply finish a run. These characters are the masked man, Merci, and the short-tempered demon, Zo. Merci has 2 HP and a coin with only two gimmick slots, while Zo has 6 HP and an enormous and heavy die which has 20 (18 available) gimmick slots and is chained to her body.
The world in the demo contains two secret shops. When you beat the last enemy in a wave, pay close attention when the shops spawn in. If you briefly see white stars shoot out like confetti, throw your dice/coin/object at it until it breaks, and a new store will appear in its place. If you break the wrong one, though, you can't break the second shop, so be sure to watch for the stars!
There are two shops, and both are guaranteed to appear once every run. The first shop can appear on wave 1-7, and the second can appear on wave 9-12. None will appear on wave 8, and wave 13 is the boss stage. Both have different quirks, and I'd definitely recommend looking out for them.
The first shop is the Rotten Playroom. This shop is run by the freaky, pink-haired Hapoten. She actually gives away goodies for free, much like the crows. She can give away objects and gimmicks, some of which I think only appear here. However, she can also have a slot machine set up, which does cost money to play. This isn't nearly as good as the freebies, ironically, since it usually has lame prizes from my experience.
The second shop is Tail End, run by the cute, yet disturbing Nikola. Nikola doesn't take money, he takes pain. By that, I mean that you have to take damage to get his items. He tends to have some pretty good stuff, though, so it may be worth it if you've got health to spare.
Ever wanted to play ActRaiser on crack? Just finish a level 10 game in the catacombs and take your reward to the end of the run. Once you've beaten the boss, go to the ending screen, grab the object you won and take it into a little clearing to the right. You won't be able to finish the extra game in one go, though, so rinse and repeat to see all of it. It kicks you out whenever you finish or fail a level, of which there are four, so if you're good, you can see it all in four plays. I'm told you can't play anymore after you see the ending, so enjoy it while you can!
Whenever you click on Hapoten in her playroom, she'll get just a little bit shorter. If you keep clicking on her, she'll get as flat as a pancake before going back to normal size. However, if you click any three lightbulbs until they fall off the strings, then click her until she's flat, when she starts to rise back up, she'll be even more messed up than usual. Once she's tall enough, she'll do a creepy laugh and run away. You can still claim your goodies or play the slot machine after that, though...
To see this easter egg, you'll need the Killer Doll and Milkman Delivery gimmicks. Splash some milk on the floor and walk over it while holding a plush. The plush will soak up the milk. Throwing it like this will cause it to deal more damage and leave additional puddles of milk wherever it lands. Additionally, you can set the plush down to suck the milk out of it, leaving it looking all deflated.
Mechanical Carp is a gimmick that hurts you. That's all it does. However, if you die due to Mechanical Carp, you'll get a strange secret ending in the form of a bunch of text about... um, carps. This is apparently a reference to an ending from NieR:Automata, so if you've played that game, you may get more out of this secret than I did, but hey, it is what it is.