Gravel's ability scores are as follows:
|9 (-1)||14 (+2)||12 (+1)||18 (+4)||12 (+1)||11 (+0)|
Gravel is a sixth-level artificer, with alchemist as specialization and the criminal background.
“Gravel? Dunno. We found him working for some two-bit gangster in Blackbottom. Same place we found Sweet. Clever little fucker. Long as he can mess with his experiments he’s content...and we get to use the shit he cooks up. Just don’t fuck with him. He’s meaner than an owl bear.” 
Behind the Scenes
Although Phil planed to create a plain human thief for the Heist team he finally decided to change to a less orthodox character.