Come to think of it, the Motorola M1225 trunking model is Part 95, I believe.
I think trunking on GMRS, while technically possible and not terribly difficult, would create a lot of usage issues for existing users and would probably end up being in violation of one or more operational Part 95 rules because of that.
There's virtually no GMRS activity here where I live, so it'd probably never generate a complaint or any interference. The big question is who'd use it. But I don't think it would fly very well in well populated areas.
And, as Dave pointed out, GMRS licensing rules do limit who could use such a critter anyway.
If nothing else, compared to simply setting up a repeater, a trunking system would be beacoup expensive.