why Kings gambit never used in pro chess?

i haven't seen professional chess player use kings gambit.

I have. This game with its annotations was published in Chess Informant 109. There are more recent GM games. In the past few years, the King's Gabit has been played by Nakamura, Vachier Lagrave, Ivanchuk, Morozevich, Naiditsch, Rapport, Grischuk, Shirov, Kamsky, Short, Judit Polgar, and other strong GMs.

Also see Carlsen-Aronian in yesterday's Norway blitz round 5.

Spassky was a protagonist.

The king's gambit just doesn't give any advantage to white at the GM level, black is probably a bit better. Of course it doesn't mean that no GM uses it for example as a surprise weapon ocassionally. But as a main opening, it is simply bad at their level.

IM David Pruess plays it.

Kasparov just used it at the St Louis tournament

And quickly got a lost position, even if he saved the draw in the end...

It's because, and I'm going to be completely honest here, Bobby Fischer thought it was a bad opening.

Interestingly enough, in double King's pawn openings, at the 2500+ level, the King's Gambit has the highest win percentage of all 2nd moves for white.

It's played a lot less than, say, the Ruy Lopez, but when it is played, the King's Gambit usually wins for white.

I wouldn't go as far as saying that this means the King's Gambit is stronger than the Ruy—likely, it just means that most GMs don't spend a lot of time preparing against 2.f4, as the chances of it appearing are slim.

Fischer lost to Spassky (before their world tournament match) in a King's Gambit, looked around and found what he thought was "A Bust to the King's Gambit," as he called his article about it -- but only "busted" the King's KNIGHT's Gambit, even if his line was as strong as he thought it was.

Later Fischer played the King's BISHOP's Gambit against Larry Evans and won. 

At grandmaster level, from what I read, almost any gambit is good for a surprise win now and then, but only good for equality or even slightly worse for the gambiter who faces expert defense.