The Gullfstar 50''s were a real mixed bag. First of all they offered a lot of performance for a pretty reasonable price. They are comparatively small boats as 50 footers go. The glass work on the hulls was comparatively heavy, but nothing to write home about. The interiors vary pretty widely on these boats from reasonably well built to slapped together to go into the charter trade and be trashed after 5 years. As built the systems and harware on these boats were really quite junky but the good news is that many of these boats have been carefully and lovingly restored and upgraded.
I think that their current inexpensive prices come from their lack of deck and interior volume (for a 50 footer) and the market perception that these were not all that well constructed when new and it had been 20 plus years since they were new.