This is a good question.
Files Gallery is already included with X3 as a bonus-feature. I call it "bonus", because it still operates as a separate application and usage is entirely optional. Also, the license is only included with X3 when used with the X3 "content". Read more:
https://www.photo.gallery/docs/plugins/files-gallery/
With login guest/guest, used as an alternative to the X3 control panel:
demo.photo.gallery/files/
Optional public view, pointing to the X3 content or a subfolder:
demo.photo.gallery/files-public/
demo.photo.gallery/files-single/
In the future, the idea is for Files Gallery to replace the current X3 control panel, because it is faster and more intuitive. I also have some vague ideas to use many of the gallery concepts in Files Gallery for X4, although I have no timeline for that yet.
Cedric is it possible (in the near future, or it is already possible) to use Files app in photo.gallery (instead of the current photo reader)?
It's not clear what you mean "use Files app in photo.gallery"? Any reason you can't just use one or the other? Would be good to know ...