I know several professional designers who only (or mostly) use free software.
Gimp inclusive.
They do this as a matter of choice.
And there are many more professional designers, who use free software for parts of their workflow. They do it, for many different reasons, but mostly because it just happens to fit their needs.
Some of them use Gimp.
If you would have read the article linked above, you would know that it's mostly about a professional designer (Aryeom) who is using Gimp for her work on an animation film.
Personally, I still believe that you're question would have deserved only a "42" type of answer.
"Professional designers" is such a vast term...
But, hey I'm so generous today :-)
P.S.: I don't want to get into a discussion on who is a professional designer and who is not, so I avoid putting links to the ones I know are using only/mostly free software...
But here are the names of the first ones that comes to mind: Cédric, Elisa, Vlada, Camille, David, ...