The traditional meaning of the Seven Deadly Sins is at odds with modern interpretation. Gluttony, the Sin Gula represents, is not limited to food, rather over indulgence in anything.
Gula is over indulgent. He's also the most relaxed of the Sins, prone to cracking jokes and least likely to take anything seriously. His role among his siblings is that of the peacemaker. He typically gets along with everyone, even the perpetual grumps like Campbell and Ace, though he does tend to annoy the crap out of them.
Gula is fiercely loyal to all his siblings, to the point where he helped Ava escape when he thought she might face death, and assisted Luxi in a task that might have ended both their lives. Yes, he might tip the odds if he feels one of his brothers or sisters is in danger, but he does his best to walk the middle line. The thing to remember about Gula is his easygoing nature doesn't mean he's a pushover. Give him something to believe in, something to fight for, and he can either become a formidable ally or your worst enemy. Test his loyalty once, and it might be the last thing one ever does.
*denotes the book in which Gula is the lead character