Basically, the study divided 80 Swedish students into 3 groups.
Gamers who play 5+ hours / week.
Gamers who play less than 5 hours / week.
The gamers did far better in English tests than non-gamers.
At that age, girls tend to be better at languages than boys.
But the gamer group playing 5+ hours / week, included only 1 girl, making the results even more surprising.
I can actually confirm this study, my English is excellent, but it's not my native tongue. It's mostly due to playing MMO.