Vincent Diepeveen <diep at xs4all.nl> writes:
  Is it allowed to use the code from GMP for that?
     Answer = yes, but if you do that then you must also publish your
     code.
  
This is a misunderstanding.  You do not need to publish derivate
works.  But if you do distribute them, you have to publish them under
the (L)GPL.
-- 
Torbjörn