Further:
5. Settings don't save between runs.
6. I realized you can interact with certain objects with E. But there is nothing that really makes it clear? Maybe you could put huge wall switches like the ones in Doom. Or even lift a texture from Half Life 1 or 2 - the elevator buttons maybe?
7. You cannot hit enemies that aren't activated. Bullets straight up don't work and do grey puffs like on dead objects. You need to first activate them to be able to kill them, which is weird.
8. There doesn't appear to be vertical autoaim. This is a key feature of Doom - because you can't aim up or down, the game aims up or down for you. If enemy (or anything damagable like explosive barrel) is anywhere in middle line of the screen, the game will automatically aim at them. This doesn't have that, which makes climbing stairs very precarious since I can't use explosive barrels to clear out high enemies.
9. Not really a glitch, but maybe hitting enemy with saws from gravity gun could be more satisfying? You have programmed gibbing - maybe just make it deal enough damage to gib basic zombies?