I really like (and own) GPO too! I also own JABB and COMB and WORLD. But I own projectSAM Orchestral Brass as well as Symphobia.
The projectSAM Brass sounds in GPO are generally great! However, things like stopped horns and certain articulations aren't in GPO. Additionally, even though I love the projectSAM horns in GPO, I think the horn sound is better coming from the projectSam Orchestral Brass. I will often use the trumpets from GPO and the Horns from SAM. Sometimes I layer both of them. projectSAM's Symphobia is an entirely different thing which we can discuss a bit if you want to.
However, as many here have mentioned, I am more of a music guy than a techie/audio guy. Audio has been very hard for me to "get" and I still struggle with it.
The instrument lists/articulations lists are available for free on the projectSAM site. But please let me be clear that GPO was my first love and I still love it and use it.
In theory there is no difference between theory and practice, but in practice there is.
ProjectSAM has quite a few more articulations than the SAM brass patches in GPO. You have more control over staccato's, legato, etc. The extra articulations come at a price of using more memory and more time spent changing the articulations.