SPOILER:
I refuse to believe Gibbs would shoot a very close friend for the death of someone he knew for four months doing undercover work. the only way I would say yes is if Fornell killed someone like Ziva or Tony which I doubt. gibbs also wouldn't kill him cuz Ziva, Tony, and McGee resigned cuz they did that for him.

If he really did kill Fornell there must've been a very good reason to that we are unaware of. But think we all thought Ducky was dead and he was fine. The gunshot was probably something else

#FreeGibbs

Reference Gibbs/Ari episodes where Gibbs and Fornell are talking in Gibbs basement about Ari and Gibbs shouts that 'Ari crossed a line. You don't shoot friends to build your chops!' To paraphrase as I can't remember the exact words. Ari didn't even kill Gerald, just shot him in the shoulder and put him in rehab for a year, I think. Gibbs obsessed on tracking Ari down.
I don't think its possible that Gibbs could have shot Fornell. Not unless they had it agreed upon.
Like in Firefly where Captain Mal was robbing the bank and the bank guard just opened the vault to them and they were trying to decide on where to shoot him at, someplace that looked like it was serious, but not serious enough to kill him......LOL
But, the writers of the show are going to have to do a lot of explaining if they let Gibbs shoot Fornell after those early episodes and how Gibbs reacted to Ari.....

No, I don't think so.