Was the gallery unlocked before you updated? it should be in the same state it was as before the update, just like the rest of your saves. The flag to unlock the gallery is part of the persistent file, so that's the one you'll want to make sure is intact. It's also what saves your various game setting such as fullscreen/windowed and text speed settings. not sure on the specifics but there's two copies of your saves. One is in:
(base game folder with the .exe)\game\saves
and the other is in:
%appdata%\renpy\MyPigPrincess-1630801825
you might want to backup a copy of your save files if you are updating, but again, unless there's something in the update that breaks the old saves (such as perhaps if the developer renames the specific folder in appdata or something maybe), then you should be able to just extract the updated game over the old one and launch it. (this is of course assuming you're using windows. if you're using mac, then the save folder would likely be in a different spot, but I'd expect a similar sort of extract update and run simplicity)