Can you still get TF2 items from beating assignments in the GOG version? I'm talking about these three achievements:
Moustache Research Assistant
Beat the assignment "Moustachium 602".

Director of Moustache Research
Beat the assignment "Moustachium 604".

Moustache Scientist
Beat the assignment "Moustachium 608".

I'm going to guess no because it's tied into Steam's inventory and achievement system.

I'm throwing out my only guess for this to happen. I have this game on steam and GOG, and they both save into the same exact folder. Maybe playing the GOG version to meet the achievement criteria, load up the save on steam, and the achievements might pop on your account to get your items. Not even sure if that would work and it sounds counter-productive.

i bought it somewhere else, but not on steam. I'm half way through and have about 75% of the achievements but no fishcake yet