Who said that each challenge was an actual battle? You've seen them do absolutely stupid things like Rock Paper Scissors and racing (in the anime)
If they both went into a full out match almost 100 times, Kakashi would be hospitalized for the majority of the series due to sheer use of the Sharingan, especially since Kakashi wouldn't be using Raikiri on Guy
It's pretty likely that it was intense sparring at best.
Read the rest of my post.....
"All 8 gates released is a 1 shot thing if he misses (which is entirely possible now that Kakashi can warp to other world) he dies. 7 gates gai will also damage himself the longer the match last. Kakashi is known for diverting with clones, analyzing opponents, and keeping a distance with ninjitsu. "
Now you can have an opinion thats fine but the fact is Kakashi is a very versatile tactician and Gai is a very powerful one trick pony. Kakashi has Obito's sharingan That means he can read and predict movement patterns to anticipate where tiejitsu attacks will land, reacts extremely fast, excels at tiejitsu himself, heck might even be able to open a few gates himself (1,000+ copied techs.) then combine this with spamming Kamui that in this case does not even need to directly hit Gai and you have a fairly even match in Kakashi's favor.
But that is not all Kakashi can do he could implement moves like the lighting clones to a devastating effect, I mean all Kakashi must do to win is doge Gai's attacks and wait for the tech. to damage Gai to the point of near death.... While Gai must make sure all his own attacks land because he has a sever time limit to beat kakashi. Then lets just say that he dose somehow find the real one.... all he has to do now is teleport away (lame I know but a win is a win)
Then Kakashi has Long, Mid, and Short range attacks/ defense. While I am sure Gai can easily doge these, he is on a time limit and constantly dogging takes time. He could probably power through a Ninjitsu but at a sever cost then he is assuming that this Kakashi that is attacking is NOT a clone.
In short this type of battle plays to Kakshi's strengths. I see nothing at all that would suggest Gai could win. (Not saying he is not powerful just that he could not win vs Kakshi)