Hi,
I live in a fairly big city with plenty of GIs, but choose to go to a GI who's about 25 minutes away (I know not really a comparison to 90 minutes, but still a bit of a hike). Long story, but while undergoing a diagnosis for something else, I got fed up with local GIs who told me to wait it out and went to this other GI in desperation on a co-worker's advice and have stuck with her since. Anyway, back to the topic 
....she and her staff are aware that I travel a distance and they're more than happy to manage from a distance, especially when things are going well. I call in when I need a script refill and they'll phone it into the pharmacy. If I'm having an issue, I call and get a phone call from the GI that afternoon. If she wants blood work or a stool test, she'll fax me a script and I'll bring it to the local blood draw station (they'll give me collection cups for stool cultures too). I'll call in with reports of how a new med is working. You get the idea ... we've managed to make it work over the last 9 years. I still see her every 3 months or more if things are active or every 6-12 months if things seemed to have calmed down.
Good luck with the move.
Melissa