I have never worked glossy, only common(European) buckthorn. It grows like a weed here!
I would cut anything bow quality you find because glossy might just be the new osage!
I know that when cutting R. cathartica I try to cut the ones wuth the least amount of spiral I can find. It would be nearly impossible to tell from the bark if a tree is spiraled however, all buckthorns that I have cut have one or more flat sides to the trunk which if you look down the tree, will follow the long. grain. Buckthorn is one of the most difficult woods to dry without checking so if you cut the trees in summer, debark them and gett some good coats of finish on the back and reduce them to just above bow dementions. do the same if cutting in winter. Hope this helps.
Cody