I suffered a similar injury 33 years ago and waited 30 years to get it taken care of. My knee is pretty much shot now and replacement is in my future.
Gallardo will be shut completely down (not allowed to bend his knee at all) for about 2 weeks following surgery. His rehab will start with getting range of motion back into his knee and then strength. The rehab should take about 3-4 months, and baseball activities could begin a month or 2 after that, once his conditioning and strength have returned.
Its likely he will not be ready to pitch again until this fall in Arizona or one of the tropical leagues, but I'm not sure I'd let him do that. He could use the entire winter to rest and work on his conditioning and mechanics,