Are there a typo in the text on this mission card? It says "If successfull, either gain 2 loyalty in this system or..." Is the "2" wrong or "system"?

No, its correct! If MonCal is imperial loyal you can turn it over to rebel loyalty by gaining 2.

As Bantha says--the card is correct. If the Mon Calamari system has an Imperial loyalty marker, then gaining 1 loyalty (e.g. with the 'Build Alliance' mission) would turn it neutral. But 'Support of Mon Calamari' lets you gain 2 loyalty, so the system goes from Imperial-loyal, to neutral, to Rebel-loyal.

Note, though, that if there are Imperial ground units in a system when you make it Rebel-loyal, it will become subjugated if it isn't already (you place a subjugation marker on top of the Rebel loyalty marker). It's still an 'Imperial system' in that case--the marker on top determines if it's an Imperial or Rebel system. But it's also now a 'system with Rebel loyalty', which may help with scoring objectives. The two phrases mean different things, so watch out for that!

That is even in the rules, page 8:

"When a player gains two loyalty in a system that has his opponent’s loyalty, he removes his opponent’s loyalty marker from the system and then places one of his own loyalty markers in the system."