There's a demo on the Steam page for the game, that's the reason we probably don't have a trailer for it, yet, as the demo is still an alpha and likely not representative of how the game will end up looking (I still remember the 2007 demo for Owlboy and how different it ended up being from the actual game, despite character sprites already being pretty much final). That's also the probable reason why there's no trailer yet: all the "gameplay" videos are 10-ish Youtube videos of people going through the demo (some of them quite poorly, I might add...).
Having transferred and played said demo, the early stages of the game already look extremely promising. The pixel art is gorgeous and the gameplay is tight, nothing too fancy, just your good old 2D action platformer, very reminiscing of the SNK Metal Slug titles. My only gripe with the demo is that I don't have a gamepad and the keyboard controls are not rebindable and the default mapping kind of sucks. Then again, that's just an alpha demo, hopefully they'll let us remap keys when the actual game fully releases. Wishlisted and added to the notifications here on GOG, for sure.