From Ren'Py

Unlock a CG Gallery

When you've decided the player has won the game, run the line:

$ persistent.unlock_gallery = True

Then change the line in gallery.rpy that reads:

config.main_menu.insert(2, ("CG Gallery", ui.jumps("gallery"), 'True'))

to read

config.main_menu.insert(2, ("CG Gallery", ui.jumps("gallery"), 'persistent.unlock_gallery'))

And the gallery extra will unlock once the player has won the game.