Gull was written by me, Adam Cadre. However, I pretty much just restated what I learned about this stuff from other people. Lots of people explained things to me for section one, including John Cater, David Glasser, Stephen Granade, Iain Merrick, Dan Shiovitz, and no doubt many others whom I'm forgetting now. The material in section two, on the other hand, I learned almost entirely from Andrew Plotkin, both directly (through asking questions on a Wiki Web set up by David Glasser) and indirectly (by reading the Glk spec.) Further comments were supplied in the editing phase by Ross Raszewski and Evin Robertson.
The subject of Gull is Glulx Inform. Glulx is by Andrew Plotkin; Inform is by Graham Nelson. This is mentioned in passing elsewhere in Gull, but this seemed like a good place to draw attention to it.