Travel along the road from Domnann Grove. Just after you cross over into Callte Garrran travel South to the river.
Many River Gobbler spawns can be found the length of the river. Goby is a smaller than average River Gobbler.
Keep killing River Gobblers from the spawn closest to the zone crossover into Domnann in the river and Goby will eventually appear. He does not drop items every time he is killed.