The rules of both GMRS and Amateur Radio prohibit communicating outside their respective frequencies except in emergencies. SImply put, an Amateur Station can not listen to a GMRS station, and the GMRS station listen to an Amateur Station for the exchange of messages (traffic) by each transmitting on their respective frequencies; aka cross service communications. It is perfectly legal for a GMRS station to exhange traffic with another GMRS station, and then if they are also licensed in the Amateur Radio Service to forward that message across Amateur frequencies; also a message from a GMRS operator can be verbally or written passed to an Amateur operator for them to retransmit the message on Amateur frequencies. All assuming the message content does not violate the rules of their respective services.