Very fun game, I really liked what i played. I ran into quite a few bugs, but I'll provide feedback on the game based on what I feel is the intended gameplay experience.
As a concept, this is very interesting, I've never played anything like this before, which really blows me away, Game jams typically breed this kind of creativity, and your game is one of the reasons I like participating in these, so thank you for your submission!
I really dig the little music you have going on here. It's catchy and not annoying, which for this little arcade experience is perfect.
The pallete you chose is very appealing, and the sprite work is well done. It's abundantly clear where enemies are, where i am, and where i'm about to place a tetris piece, my reload time, and where current tetris pieces are. The height of the pieces placed is pretty spot on too, there isn't anything in the art department i would really change, so superb job there!
Placing tetriminos is satsifying, the nice "chunk" sound when you put them down is really great.
This concept is fantastic, An arena shooter, where your gun is placing tetris pieces down on top of enemies, which in turn makes the arena smaller, so you have to be super smart about your placement, kiting enemies, and all the regular stuff, It's really great, I really really like the idea you have here.
I think if you just iron out the technical side of things, You have something really special on your hands. Well done!
And of course if you can take a moment to play my submission too, I would greatly appreciate it.