Where is the Gear oil on 2012 malibu 2.4L, and which oil is suitable for the Gear? and how to change?

Are you talking about a manual transmission? On a manual transmission there should be a drain plug on the very bottom of the transmission and the fill plug about half way up on one side. You'll need to find out what weight gear oil is recommended by Chevy. It could be 80 up to 120.

Thank you Ytlas, actually I was asking about automatic transmission, does it is the same position as manual? for the weight is OK, but I don't know which type of gear oil is specific? thank you again

No. On automatic transmissions you usually have to unbolt the entire transmission pan to drain out the automatic transmission fluid. Unless they changed things on your car, the automatic transmission fluid is usually filled in the same hole where your dipstick is. It's a very messy job. This should give you an idea http://www.wikihow.com/Change-Transmission-Fluid