I'm sure you already know about all of the scripts already mentioned in
this thread, so I'll just mention that I'm working with the new
LinkX script from JMB and it does give you the ability to assign multiple categories to one free site (or gallery in your case). LinkX has a bit of a learning curve, but that's no different from any of the other link list scripts I've checked out.
If you don't need all the features of LinkX, then JMB's AutoGallery SQL script might work for you too. With that one you can't assign multiple categories to one gallery, but you can create category pages which will pull galleries from several categories using both category and keyword information. Actually you can even do that with TGP Rotator if you use the SQL "WHERE" condition to select the galleries/sites to list on each category page.
HTH