Licence question...

If I create an executable with "gplc" consisting of which is both pure Prolog and custom "C" interface code, under the terms of the GNU licence, am I at liberty to sell that executable as a commercial product ?

If not, why not and what would I have to do to remain within the rules ?

I have read and re-read the various GNU docs etc. but not being a legal eagle I am afraid it has all left me feeling rather dazed and confused as to what is and what is not permissible etc.

I certainly don't want to "fall foul" of such a wonderful community and should my "product" ever see the light of day I'd be deliriously happy to be able to one day be in a position to fund some proper developer time to boost the progress of GNU Prolog… indeed that would be awesome!

Thanks all,
Sean.