The info came from 2 of the engineers responsible for the creation of the Arctic Cat line. They were there and worked for both AC and GL. That being said, they aren't spring chickens and one had to ask the other to confirm. This is all via email tag with one of their sons so I didn't get to ask a follow up about GP. You may be right.
I think all that is fairly certain is that GA came somewhere before GL. It's possible GP was the transition but that wouldn't have left much time between GA and GL, only a year or two. Without dates of the GP Trail Horse models, it's hard to tell.
I bought Arctic Cat bike parts from a place called "General Parts" in Omaha in 1979 or 1980. It was just a tiny parts place, probably just the leftover remnants of a former larger company. Maybe the stock and the name were bought out. However, Bob Fragoman bought the majority of the remaining AC parts and the name. He's next on my list to talk to. He'll probably know.
Do you have any mfg. dates for that Trail Horse in the pics? Any dates or clues in the brochure you found showing that bike? It's an interesting mash up of a 1971 AC Climber frame/swing arm, SSSCat tank, and Trail Horse forks. Because of the tank mount style, I'd think it was made in late 1970, early 1971.
Since I live in Omaha, maybe I just need to go dig up some old city records to see who came first, second, and third. My dad remembers where GA and GL were located and the building is still there, but he didn't recall if GP was ever there. He's old too so that's not saying much. The plot thickens! LOL!!