Jake Gyllenhaal -- Scary Jacked ... For New Boxing Flick

Jake Gyllenhaal is a beast.

The actor got insanely ripped for his role in the upcoming "Southpaw" flick ... a movie about a boxer who loses everything and has to fight to get his life back.

The movie looks super emotional ... and 50 Cent has a role in it ... but the headline here is that Gyllenhaal looks like a real-life fighter in the new trailer.

#respect