I am try to make a BF2 map like BF1's Attack of the Pigs by Era of Desann, but I can never get the Gamorreans to work. How do I get them in the map?

Put this line in your .lua to load the Gamorreans from the GAM side.
Then replace one of the units with "gam_inf_gamorreanguard."


Also, try searching next time before you post. This type of question has been answered several times before.

Is it okay if I try this idea too?

Sure, you can do it. I probably won't get it working anyway - I'm not much of a modder!