Um in the chuunin exams when shinobi go up against each other remember gaara vs rock lee. Well when you win it means you pass to the next stage riiight. The time when Obito was having his flashback to the chuunin exams he went up against gai sensei. As we all know gai won but then he was put up against kakashi and he lost. Dont you think he was screwed
He's trying to say Gai was owning the chunin exams, but going up against kakashi just sealed his fate.
In other words going up against kakashi screwed him over.
At least that's what I think he means.