In FSDS you have the option to move vertices in the right upper corner in lets say the front view and automatically the same happen in the left upper corner in mirror reverse. So I can push vertices in the right half and exactly the same happen in the left half. How do I do that in Gmax.I am referring to do it just after I have created the fuselage from a cylinder, before I have split it in half.

I don't have GMax loaded at the moment, but if it has the Symmetry modifier available, try adding that to the stack.