Gelbugs are creatures native to Bryyo. They are encased in Phazite, and both feed on and are partially composed of Fuel Gel. Because of this, their bodies are very unstable, and can be detonated by weapons fire. Based on scans, they are most likely the adult form of the Shelbug.
"Gelbugs are roaming insects encased in organic Phazite. These cavern dwelling bugs have come to rely on Fuel Gel as a source of sustenance. Over time their shells have become saturated and crystallized with Fuel Gel. As a result, their bodies have become highly unstable. Weapon fire will disrupt the gel and cause the creature to detonate."