Gasbags on the Fjordur dlc for Ark Survival Evolved are not found on the main map, they are found in the Asgard realm. To travel to the Asgard realm you need to use the Portal Cave, or hold down the R key on PC to get a menu of areas to travel to. Once on Asgard you will find Gasbags on the open plains. I have found them in these general areas.
Fjordur Gasbag Coordinates