Gaia004: Not rushing. Will you notify us somehow when it's done? Thank you.
It's in now actually, and will be in the 1.0.9 patch as soon as it's live on GOG! Sorry for the delay on this, it had fallen down my task list again due to some more serious issues and because I really didn't want to delete everyone's achievements by mistake!
So, to do this you're going to have to do a bit of options file editing... You can find your options files in one of these locations, named SaveA.options (or b/c/d depending on which slot you use).
Windows: %USERPROFILE%/AppData/LocalLow/Dodge Roll/Enter the Gungeon
OSX: ~/Library/Application Support/Dodge Roll/Enter the Gungeon
Linux: ~/.config/unity3d/Dodge Roll/Enter the Gungeon
You'll want to open this file and add a line somewhere in the middle that looks like this:
"wipeAllAchievements": true,
Next time you load up the game on that slot all of your achievements should be reset. Note that you may have to restart the game once or twice to get the state on GOG Galaxy to update. We have a lot of catch up code that runs when you start the game to unlock achievements we can see you should have earned based on your save file, so if you have a lot of progress already you should get many of the achievements you've legitimately earned unlocked right away.
Let me know how this goes for you all, and thanks for your patience!