Hi, Fantastic--I'll take at your first two questions, leaving the Logic info to a Logic user.
"...Does the GPO 4 allow a note that is being held to become louder by pressing down on aftertouch or by using the mod wheel?.."
Mod wheel controls the volume on the pitched instruments. They're very responsive to cc1, with the timbre changing along with the volume. I think you'll be very happy with the way it works.
Tympani roll - I strongly recommend that you Don't seek out a roll sample. Very inflexible. It's easy to get good rolls in GPO. A couple of tutorials were posted recently - The key is you want to Play your rolls with your keyboard, then tweak. Dang--I came here without the thread URL - In the technical Forum. I'll come back with it.