Is there a grammatically need to hyphenate the compound words "dumb f*ck" within a novel?
Used as an adjective, I believe you must hyphenate two words if they consist of an adjective and a noun. For example: starry-eyed, fast-acting. Here, the word dumb is an adjective and if you're treating fuck as a noun, you could also hyphenate it. For example: He is a dumb-fuck dude.
Used as a noun, it looks good to me when you join the two words. For example: He is a dumbfuck!
In your novel, it would be:
"It's been six years, you dumbfuck," Gus muttered under his breath.