This GDD immediately gives me the impression of a fun wave survival game mixed with a tower defense, which isn't exactly a new genre as it's not really reinventing or contributing anything peoplehaven't seen before.
There are a few sections where concise clarity would help improve readability and getting your intent across better, most notably sections where the word maybe is used a bunch.
The story and gameplay elements that are described seem fun and where most of the time and effort where placed but I kind of wonder how that really meshes with a few elements that tie in with each other. If this is supposed to be an area of defense to hold back the aliens against humanity why are they starting off so under prepared with a single turret? Just something that could use more thought or polish and could be gone into more detail for the setting and setup which is a rather easy fix, seeing how the story elements you do have are rather solid. Another thing that struck me is that the game is called One Gun but during upgrades you are attaching multiple guns to hard points so it's technically more than One Gun? Just something funny that confused me on that aspect.
On the whole I would play the heck out of this game, always into upgrading systems and growing big and strong in games and mowing down unending enemies.