Use the Pack of Gnar Thumpers to locate a Burrowing Gnar. Gnar will NOT be found in the water and seem to only burrow on the sandy beaches.
Once discovered, the Gnar must be engaged or he will walk a short distance and de-spawn.
Once a spot has been used there is a cooldown before a Burrowing Gnar will be found there again. Move around the area and repeatedly use the Pack of Gnar Thumpers to find them all.