The way you wrote this is perfect colloquial English. Just add the comma (in red below).

World cup

Which country is gonna win the world cup, Germany or Argentina?

However, if you wish to write the same thing in more formal English, you should write:

Which country is going to win the world cup, Germany or Argentina?

Personally, I don't care very much who wins, although I have a slight preference for Germany because my maternal grandfather was German.