Wow! I do see your issue now, after viewing a forum message, I clicked the Forum Index link and received the blank page. Very strange indeed, enter the URL directly, works fine.
Can you see the webserver's error log. Anytime I have had an issue like this, there are usually good clues there (normally I'll see the actual error message generated by PHP).
Besides loosing the gl_users and gl_userprefences tables, were there any other items that were corrupted, lost, damaged? I'm actually curious how only those 2 tables could be damaged. Anything else on the site changed in this process?
Rebuilding from scratch will be a challenge, but I'm sure you can pull it off, I'm just wondering if you backtrack over what caused the initial problem with the tables and any other changes made if that might give a clue to the issue. I see the forum plugin has issues, but browsing around in gallery and the classified ads seems to be OK.
These are the kinds of challenges I like to see solved, not sure I'm much help.