Grump is not a Gronckle. He's a related, but different breed of Boulder Class dragon known as a Hotburple.
Like a Gronckle, the Hotburple also makes lava slugs out of molten minerals, but he prefers iron ore over the Gronckle's love of rocks. As such, he eats the scraps around Gobber's stall and keeps the forge fired up with molten loogies. Personality wise, he's like a walrus -- a loud, fussy, lazy lay-about who snores obnoxiously and easily falls asleep at any given moment, even while flying. He's always in the way in Gobber's stall and he's the perfect match for Gobber's put-upon attitude. They banter like an old married couple. Otherwise, he's a lot like a Gronckle in most other ways.
See more of Grump in:
Dragons: Race to the Edge
How to Train Your Dragon 2
A loyal, lava-launching helper -- when he isn't napping
Heptane/Oxygen + Rock = Lava Attack
Shoot flaming chunks of rock/lava