Probably sign up for the training classes would be the best way. It also depends which version you have. The newest releases have a Cad-Modeler which requires full AutoCAD to be installed. You can draw up a frame in ACAD 3D and run the analysis in GTS. Their older user interface is GTMenu which takes a little time to get used to. The old timers use the command line interface and edit the input file as they go to get the exact model you want and control the joint and member numbering.
It depends on what type of models you plan on doing on how to master it. I've been using GTS since the 1980's and pick it up about 6 times a year to do some projects, mostly open industrial type frames and I still have to contact tech support to get tips on how to use some options.
_____________________________________
I have been called "A storehouse of worthless information" many times.
