For GAMS on Unix (We did our experiments on Linux but are pretty sure that the other UNIX systems behave the same. If there is still doubt, please let us know what Unix you are running) CPLEX returns CPU seconds in resusd if Cplex runs sequentially (which is the default unless you specify option threads, mipthreads, and/or choose the concurrent optimizer).
If Cplex runs in parallel (option threads or mipthreads) the resusd is real time also sometimes called wall time. If you run sequentially, you should get always the same CPU time to solve the model. If you use parallel, you get wall time and that depends on the overall load of the machine.
Everything said also applies to the PC (Windows operating system) with the exception that even if we run sequential Cplex, we get real time, not CPU time. This seems to be a tradition on the PC.