This isn't a fic, I guess it might be a longish drabble. Probably not even that, just a little bit of trying on Gai's headspace for a while. :) Takes place around episode 86 so spoilers if you haven't seen that part.
And my take on Gai probably isn't what you might be expecting. It isn't funny, for one. ;)
Nightwatch
It was well into evening before Gai returned from delivering Sasuke's unresponsive body to the infirmary. He'd stopped to see Lee, and of course the boy was shocked to learn that Sasuke had been bested by his brother, even though Gai had explained it clearly. A Sharingan-user of extreme genius-level ability - Itachi had been far in advance of either Sasuke or Neji at that age, and had doubtless spent his time honing the twisted uses to which he put his skill. Gai had faced the man prepared to fight, when he'd come to back up Kakashi - but confidence was not an ignorance of the possibility of death or insanity. It was simply a man doing what a man was required to do in defense of his friends, his rival, and his village.
Kakashi's room was quiet, but not entirely empty. Kurenai sat in a chair, a magazine on the floor beside her, drowsing. He sent her off to have something to eat and to rest. Someone else would come by in the morning. In the meantime, he couldn't be talked out of being the one to sit the night watch.
Alone in Kakashi's room at last, the small lamp at the lowest setting and the night sounds of the village familiar and reassuring outside, Gai rested his eyes on Kakashi's unconscious features. He tried to imagine what Itachi had done to bring Kakashi of all people to this state and found his imagination failed him. He waited for a long time, half expecting the Copy Ninja to suddenly open his eyes and sit up, demanding to know what the current situation was, but when it didn't happen after several hours, Gai slowly released himself from the rigid posture and allowed himself a real slump.
After a while his mind and attention wandered, and his eyes drifted up and settled on the pictures above the bed. Two photographs, the groupings and the individuals within oddly similar. There was actually something mildly disturbing about the similarity. But there were differences too. The Uchiha in each picture could not have been more dissimilar from each other... It was obviously why Kakashi tried so hard with Sasuke. He saw his younger self there. Gai wondered for a moment if the boy realized the likeness didn't evoke a fond feeling in Kakashi. If Gai knew the man, and he knew him better probably than anyone else, it made Kakashi angry, instead.
Well, it wasn't up to him to speculate about such things. It was nearly impossible not to do, of course. Speculating about Kakashi was practically a hobby of Gai's. He'd puzzled many things out about the secretive enigma that was his rival. Not enough to keep the man from being able to get his goat with ease. But that was as much because it was a game they both had become almost dependent on, he thought. He was almost certain.
/Kakashi.../ he spoke to the man without moving his lips. /You can't be brought down by this. I can't be without my important rival. Such a thing would be unacceptable./
"I'm the only one who can beat you," he whispered.