Without resource material handy, here's was I recall about GP
Based in Grand Island, Nebraska.
Flew as an independent regional for a few years, mostly connecting small Nebraska airports like Grand Island, Kearny, McCook, North Platte, etc with Denver and perhaps Omaha.
Signed on as Continental Connection feeder in Denver in the late 80's or early 90's. Routes were expanded out of Denver, although primarily they were among the smallest, sometimes EAS subsidy, markets.
Did for a fairly brief period fly some point-to-point markets in the upper Midwest kind of like Great Lakes did for a time, including routes such as Omaha-Minneapolis, Des Moines-Minneapolis and Madison-Minneapolis, filling the cracks between Northwest flights.
Picked up some subsidized EAS flying in the southeast out of Atlanta around 1992.
Shut down roughly the same time that Continental shut down their Denver hub, although I believe they flew for a time without the CO
* designation. I think they shut down around 1994.
Flew primarily Beech 99's, although I think they also acquired some Beech 1900C's.
Again, without reference material handy I'm not 100% sure of every date and ever route, but this should give you a pretty basic idea of what GP
Express was about.