How about something like "Gravel and Trail" which differentiates the sport from any of the paved road riding and from cyclocross with its relatively short, fixed course and artificial obstacles. I think it's going to be a bit harder to draw a line in the gray area between MTB and Gravel Grinding because there are many trails which are suitable for either. Personally, I think of MTB as being off road or on unpaved trails which almost demand some form of suspension and/or wide knobby tires. If the trail is smooth enough and with a surface solid enough for a rigid bike with less than 40mm tires but you chose a MTB, I would call it gravel grinding on a mountain bike rather than MTBing. IMHO.
Gravel Grinding is a melting pot of other cycling styles, so there will always be some crossover to its parent sports, but I think it is important to recognize that it is here to stay and will continue to grow and evolve.