What the hell? Gatta just died on my game. Just after Operation Mi'ihen. This did not happen last game so why did it happen this time??

Because you have a choice when you talk to one of them and if you select a certain option it determines which person dies. If you tell him to go to the front lines then he dies if you dont the other one dies.

Oh and that was a spoiler for people who havent gotten up to that so be careful..

My Gatta also died, because I had sent him to the front..... :(
Maybe I shouldn't do that the next time.

My gatta died aswell, but I didn't know you could let him live. I'll do that next time =/

I've done both, just to try it out. Yeah, it just changes who dies and some small bits of dialogue. It also changes which you see at the end of the game. Unfortunately, it also means that neither are in FFX-2.

My Gatta is happily becoming a cocobo breader. Slightly inconsistant compaired to FFX-2 as at the start of FFX-2 he is both definatly alive and not sure what to do with his life.

You're thinking of Clasko, not Gatta.

Are there any other things like this that change the outcome of certain things i the game?

Yeah, try looking for the compatibility guide on GameFAQs for information on how to determine which girl throws the blitzball to Tidus for his Blitz Ace Overdrive and also gets some special extra dialogue.

Off the top of my heard, you can tell Clasko whether to become a Chocobo Breeder or to stay a Chocbo Knight. You can do this outside Rin's Agency the first time you visit Lake Macalania. If you tell him to become a Choboco breeder, then you can get an item (friend sphere) once you gain full control of the airship. When you do have control, get back on board the SS Liki and you'll find him there with the item.