chess, you have my sympathy.
wrt the tub surround, greenboard is NOT a code-approved tile backer either in most cases. Even the best tile installation is not "waterproof". The tile substrate should have a vapor barrier behind it, then either floated mud (pure art!) or cementitious backer unit (CBU - brand names like hardibacker, etc.). Resist the temptation to just try to patch the hole. Likewise, I hate to tell you this, but redoing a tub surround could be more expensive and/or time consuming than you think.
Good luck.
