You gotta consider that Gabe's costumes look cool, but are totally not okay for military actions. Like, everyone needs to look like they are apart of a group, and the current overwatch uniforms look like a ragtag group of heroes, not an officially sanctioned military group. I'm just imagining Reyes Making a bunch of cool costumes and then Morrison yelling at him because they don't even have the overwatch symbol on them and civilians think they are terrorists or comic nerds!
Oh I completely agree their costumes are good for cosplay and renfairs and probably not an organized military branch. Which is actually great, because it means that either
1) Gabe has seriously no idea what is acceptable workplace attire thus the later reaper costume
2) Gabe knows exactly what acceptable workplace attire is and does not give a single fuck
or 3) Gabe made the outfits for the halloween party next year but everyone misinterpreted their intention and started wearing them in actual operations, and now he is simultaneously mortified that his clearly over-the-top halloween costumes are now available for public scrutiny and elated since organizing this goddam clown car of a military branch drives the upper management insane