I have gerd. why does it feel like gas bubble come up from my stomach and get stuck in my throat?

Just a sensation: It could be just a sensation that something is stuck in your throat. This is due to having had untreated gerd for a long period of time. You should see an internist and possible a GI specialist for treatment. If you've had gerd for many years, you may need to have an endoscopy to evaluate your esophagus.

Reflux or ?: You may have a hiatal hernia, in which part of stomach comes up through diaphragm opening and sits in your chest (pain radiates upward); or might be from more severe reflux, or might be an esophagitis (from acid/meds/other). If you are on ppi (prilosec, et al) or h2 blocker (pepcid (famotidine) & co.) with persisting symptoms, should be seen by GI for evaluation & possible scope.