There's an Unlock All button, somewhere in the Gallery.
There seems to be a bug in the last version (4.3.0) where the button does not appear. I also do not see it. And the Kobold has disappeared entirely. Not that it's a problem for me as I already have most if not all things unlocked... Just saying something to say something. I'm sure the team is already aware of this issue and have already addressed the problem accordingly though. So, see you in the next update ^_^ & thanks for the hotfix!
EDIT: Can confirm. This was immediately fixed in v4.3.1 Recommend installing and unpacking the most recent version for those of you having this issue ;)
I believe the button is disabled on the Itch version. I had to hack the game scripts to get it to show up.
Also: A suggestion for the gallery itself is that whereas the tooltips with hints are a great addition, might be worth putting in the content tags for each one for additional clarity for those who are looking to unlock them on their own