In both American and British language, the speaker technically has the option of doing either, depending on whether you view the gang as a single entity, or as a bunch of individuals.
However, as indicated by many here, British English tends to prefer "were" in this instance. American English, on the other hand, is far more often "was".
The people talking about "were" as an exception to American usage to indicate acting individually are mostly wrong, except that they do have the choice to do so. Unlike them, I will cite sources below.