Gai being Gai, the habits most normal people would consider "bad" were simply habits. Or, more accurately termed, mannerisms. Without them, Gai wouldn't be Gai. So of course, Gai's bad habits were somewhat unusual as far as bad habits go.
You see, sometimes, Gai would stop talking. And not for a moment or two of lost thought or perhaps a handful of minutes caught up in one of the dark dreams every ninja acquires. Occasionally Gai wouldn't say anything for an entire day. No exclamations of Eternal Youth, no declarations of Hard Work, no calm assurances that things would be alright for an Important Person who seemed down.
And Gai knew it was a bad habit, though it made most people sigh with a bit of relief. It had made the Third sad when he was alive to see it. It made his students quiet. It made Asuma and Kurenai and the other jounins worry. It made Kakashi come over in the evening with two bottles of sake and a similarly silent presence to keep him company until the sky lightened and that silence slipped out the front door. And when the silence lifted, it made Gai remember how old they were all getting these days and how little else they could afford to lose.