Here are the two most common theories
1. Gman is actually Gordon Freeman (G=Gordon. Man= Freeman-Free) He came back from a a time device. ( I think this is a load of crap, Gman cant be Gordon, Gman was the name Fans gave to the mysterious person they first met in HL1)
2. Gman was hired by...
Yeah, there are much servers and there are many people whp play it, the latest patch (Armored Fury and Euro Force :O) convinced more people to go play BF2 again.