I can't speak specifically to where the Itch...store/program thing might put the game folder if you're using that, but once you've found the game folder, the gallery file is in www\Gallery Unlock. Gallery.js. Move that to www\js\plugins and agree to overwrite when prompted.
There should be a gallery option in the main menu regardless: the file just unlocks the individual scenes within it. If you're not seeing a gallery option, you likely downloaded an old version. Be sure you have 1.0.4 (though it's in .3 as well).
