I have the same troubles like some here with the Steam overlay running and yesterday when I tried to design my first turret something went wrong and like most I start the game without Steam now.
It seems that my designs are saved in the “C:\Users\Mark\Documents\My Games\GratuitousTankBattles\player_units” (running windows 7) but the one I tried to create yesterday is in the “D:\Games\Steam\steamapps\common\Gratuitous Tank Battles\data\sampleunits”. Maybe this has something to do with the problems?
Is there a way to obsolete old designs or is it just deleting files from windows?
Thanks for a great games.