Oh great, another broken game shoved up for everyone to laugh at.
The gun can't be equipped, there's no way to equip anything in inventory. It's not IN the inventory, clicking on the big gun on the bottom to go to the equip menu, there is no control mapping or control diagram, the walk animation is a mess, the animations seem weirdly roto'd, and I can't help but think that this was someone's very first project. Maybe someone with a public embarrassment/humiliation fetish.