I wrote Mailing Manager ST for the Atari ST/TT back in 1991 using GFA Basic. SOld that program until I sold the writes in 1996. I think it was like 16,000+ lines of code and I maxed out what you could have in a resource file (64k I think).
Those were fun times. I moved on to PC software development using VB 4/5/6 and then VB.Net.
For it's day, GFA Basic was cutting edge and very quick. Only a few things I ever did needed more speed, and I then wrote that stuff in Assembly.
Still some of my best memories were back in the day with GFA.