It sounds like you just need to rerun setup on the site and clear the site cache.
Yes do the above.
I’m sure Gallery uses hard coded paths in its tables, the gallery_items table springs to mind, have look at these, you may have to adjust these manually, setup won’t help with this.
Use MODx, or the cat gets it!
I seam to recall the paths being relative in Gallery, but if not a look in the System Settings should have the settings you would need to adjust.
If you need any help getting things working let me know.
Problem fixed. Thanks for the help!