I believe that Blue is a temporary choice of Gym leader until Virdian can find a suitable replacement,i say this because Blue is not the kind of guy to just give up traveling and become a Gym leader,Besides in the games Blue is often traveling even though he is the Gym leader.Also a Nurse Joy says that someone came in and everything and said when they left they said "smell ya later" so if Blue was to travel that far from Kanto i don't see how he could still be the gym leader.
On the flip side, other gym leaders have been seen in other regions. Sinnoh gym leaders were in HGSS for instance, and Jasmine in Sinnoh (and if you pay attention to backgrounds in contests, Hoenn too). Cynthia, a champion, has been in Unova too, which is significantly far away from the Japanese regions.
Heck, there's also the world tournament in Unova come to think of it. I suppose gym leaders just happen to have some down time every now and again.
The real question is how GIOVANNI ever got to be a League official...
$$$
Having a decent team of a ground theme (mostly) helps too, I suppose.