Can I make a suggestion? [says he, ploughing ahead regardless... ]
In practical terms, getting people to go interstate is not going to be easy - I'm not sure good intentions alone would get folk on a plane or drive for 12 hours.
So my thoughts are that 2 alternatives might be more realistic -
One would be to have more than one local GTG in each city, perhaps on the same day - that would be easy, and here it is long overdue.
The other is that to get folk to travel interstate they would have to be travelling for something a bit special - as in, we all chip in for some kind of awesome day out, like hiring a boat and skipper between us for day, alcohol and food, I don't know. If people chip in a hundred bucks or whatever towards whatever that special something might be, I think they are more inclined to travel for it, if that makes sense.
Just a few thoughts anyway.