En las FAQ de la GPL [gnu.org] habla precisamente sobre eso:
If I port my program to GNU/Linux, does that mean I have to release it as Free Software under the GPL or some other Free Software license?
In general, the answer is no--this is not a legal requirement. In specific, the answer depends on which libraries you want to use and what their licenses are. Most system libraries either use the GNU Lesser GPL, or use the GNU GPL plus an exception permitting linking the library with anything. These libraries can be used in non-free programs; but in the case of the Lesser GPL, it does have some requirements you must follow.
Re:El caso de Linux es especial
(Puntos:2)( http://www.miriamruiz.es/ )
If I port my program to GNU/Linux, does that mean I have to release it as Free Software under the GPL or some other Free Software license?
In general, the answer is no--this is not a legal requirement. In specific, the answer depends on which libraries you want to use and what their licenses are. Most system libraries either use the GNU Lesser GPL, or use the GNU GPL plus an exception permitting linking the library with anything. These libraries can be used in non-free programs; but in the case of the Lesser GPL, it does have some requirements you must follow.