Gerard Butler was in shape and quite muscular when 300 was filmed, but not anywhere near the level that he appeared as in the movie. A lot was airbrushing, 3d effects, lighting, etc.
But of course he did let himself go a lot.
I think Gerard Butler would make a GREAT action hero if he had maintained and improved his physique even beyond 300. That was what made him popular. His action hero persona and physique. With it gone...He's losing his asset.