In most cases if your welders are qualified for GMAW they will also be qualified for FCAW but only if the essential variables are the same.
As an example, if he is welding with FCAW with a certain F number, his WPQ will have to meet that F number.
Transfer mode is also an essential. FCAW is a globular transfer.
If you look at QW-404.23 you will find that for procedures the product form is an essential variable. For performance qualification, though I do not speak for ASME, just my own experience, the thinking is that the usability of the processes is close enough to consider together.
GMAW/FCAW are considered the same process by ASME. If you think about it, they are pretty close: both use the same equipment, wire feed mechanisms, etc. The other posters are correct, if you qualify one you qualify another within the same range of essential variables. So if I give a guy a test using E71T-1 (F-6 material), he can weld GMAW with ER70S-3 (also F-6 material), keeping all other variables the same. Someone mentioned transfer mode and there is a lot of talk about that. The essential variable involved, QW-409.2 states that you only need to requalify if you change from Spray, Globular or Pulsed to Short Arc and Vise Versa. This is true in any code, short arc is always an additional qualification. So as long as you are still using globular, spray or pulsed, you're ok.