How really are Chess Grandmasters become like one? Can anyone become a Chess Grandmaster? I guess yes, if you have the ability to comprehend and or process complicated chess moves in split seconds and execute counter moves against your opponent with devastating effects.  GMs are made that way, but GMs can not stay in good form if he just sits on his laurels and not practice his game, join in competitions and or study a lot.  In a sense, it is not easy to become  a Chess Grandmaster because the title erodes away little by little when the holder does not work out to maintain its luster. And it's not surprising that we maybe playing with them over here this minute using different names since chess sites like this is a haven for all chess lovers including GMs and novices alike.

wow..

I think chess is one of those skills in which you can never get rusty, like riding a bike.

why?

Well... You can get a little rusty, but it comes back quickly.  The reason?  Chess is a game of understanding.  The more you actually understand chess, the better you are at it.  Practice helps, but true understanding of the game brings glory. 

I've seen many having played in many years, then maby stop for a year, and be very rusty, not playing good the first month or so.

Of course not my friend. Chess is a game that primarily requires a lot of mental works and practice will keep one always atuned. Laying off for a while losses one's quickness in processing offensive or defensive capabilities.