If I add 12 months Gold in my Ultimate current subscription, will it reduce to months of...

You're current subscription won't change, but when you add the 12 months it will be converted to Ultimate as well, but the conversion rate is based off of price rather than time. 12 months of Gold is equal to 4 months of Ultimate. So if you add it now, you'll have Ultimate until 07/2023. If you want to use the full 12 months, hold onto the code and make sure recurring billing is turned off. Then add the 12 month Gold after your current subscription ends. If you still want Ultimate at that time, buy another month of Ultimate and it will convert the full 12 months of Gold to Ultimate.

Thanks for answering! Now the last question... What if I redeem some game pass code after redeeming the gold, would it change ou only become ultimate?

I have done this some months ago when game pass was only $2 BRL (brazilian currency), at that time I had gold already running, then it converted into ultimate, is it like this yet? (and the game pass for $2 was only 2 month subscription, but converted all
my gold period (12 month) into ultimate that I have until dec7th.)

Yes, if you add the Gold while you have Ultimate, it will convert to 4 months of Ultimate. In order to get the full 12 months, you would either need to cancel your current Ultimate subscription or wait for it to end on December 7 and then add the new code
after your current subscription has completely ended.

Is there any way I can cancel it now with no need to wait for dec7th and redeem my gold today?

You should be able to cancel at http://account.microsoft.com Just make sure the Ultimate has completely ended and you don't have any active subscriptions before adding the Gold.