Gachas do not spawn on the main map of Fjordur, but they do spawn in two of the Fjordur's realms, Asgard and Vanaheim. If you are going to tame Gachas on Fjordur, I recommend going to the one on Vanaheim. My experience there is that there are fewer aggressive creatures, plus it just looks awesome! Be aware that the Vanaheim Gacha spawns are located around the big tree, and up on plateaus. You may need a creature that can climb, a flier, or some tool to get to them.
To get to the different realms and tame a Gacha, you will need to use the terminals in the Portal Cave, or on PC you can hold down the R key until you get an option wheel of spawn locations.
Fjordur Gacha Coordinates
If you are looking for resources check Fjordur Resource Locations for more info.
If you are looking for dino and creature spawn locations check Fjordur Creature and Dino Spawn Locations for more info.