The linking of GMRS repeaters to each other is a controversial subject at best. There are many interpretations of Part 95 as to whether it is legal or not. Your mileage and interpretations may be different from mine. This is not a debate on whether or not linking GMRS repeaters is legal.
I have been searching through the Internet to find out how the linking is done, I found several organizations have linked their repeaters through a state or regional group. Most state groups I found are west of the Mississippi, and most regional groups are east of it. There are only a few organizations that exist on a national basis, and there are not a lot of repeaters listed in any of them. Everyone seems to be using the same software, which is the AllStarLink Asterisk system designed for amateur radio.
In my state, Pennsylvania, there is no cohesive system in place. You will find a few repeaters linked through myGMRS or GMRSLive, but nothing you could call a state network. It seems to be based on the repeater owner’s choice as to what repeaters are linked. In south central Pennsylvania, there was a group that had started to link repeaters throughout the Susquehanna River valley. It was a well designed system with very little overlap in coverage areas. However, it was reported to me that the FCC visited with the repeater owners and told them that they could not link their repeaters. A month later, only two of the repeaters remain on the air and they are not linked.
To me, linking repeaters is a good idea for several reasons. A linked system can help get information to the right place during an emergency. A linked system would allow travelers to communicate with family in different areas. A linked system would allow travelers to talk about various road conditions so that drivers could avoid a major accident or weather problem.
However, there can be drawbacks. One user of the system could tie up all of the repeaters and possibly prevent emergency information and traffic from being communicated to the people who need it; the remedy for this is to drop that repeater out of the system temporarily. But the positives outweigh the negatives in my mind.