A ham and a GMRS repeater can live comfortably on the same rooftop or even the same tower. If the ham repeater is an older one, there is a good chance it is a 2-meter machine, which is far enough away from GMRS that it won't be an issue. Even if it is a UHF 70cm ham machine, both can live on the same space - just as long as both of you have good quality BpBr cavity filters, not just cheap duplexers. Stop by and talk to the building owner or manager. Tell him that there is already a ham repeater up there on his roof, which is a federally licensed service, and that you would like to add an additional licensed service similar to, but not exactly the same as ham radio. Be sure to note that it is being done as a public service, and not for profit in any way.