AGT (The Adventure Game Toolkit) was created in 1987 by David Malmberg. It was a heavily modified version of Mark J. Welch's 1985 tool Generic Adventure Game System (GAGS).
It was ported for a number of platforms, such as DOS, Atari ST and Amiga. Read more about it on Mark Welch's blog.
(Because of the compatibility of GAGS with AGT, GAGS games are listed under AGT in the CASA database. Games with a release date prior to 1987 can be assumed to have been originally made with GAGS)