I think here is way Rockstar made a slight error! The stadium was used for Football (or Soccer as it is in the US) in 1998. It was then converted to host (American) Football... But the Liberty City Cocks played the Liberty City Beavers in 2001 according to Liberty Tree and they're both soccer teams!

So? The New England Revolution (soccer team here in New England) play their games in Gilette Field, which is also home of the New England Patriots (a football team here in New England). As long as the field is big enough, both footballs (soccer and American football) can be played there.