Can Paladin's become grandmasters in longswords? If not in BG2 how about AD&D?

That's a negative. Paladins along with rangers cannot hone their weapon skills beyond "specialized", which is two points spent on the proficiency. According the the 2nd Edition AD&D rules "Weapon specialization is an optional rule that enables a fighter (only) to choose a single weapon and specialize in its use". So that excludes rangers and paladins. Hope that answers your question.