The GNU GPL permits a distributor to "charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee." This is further qualified by the statement that the distributor must release "for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code." In other words, the GPL ensures that programs like Linux will at best be free of charge. At worst, you may be asked to pay for the cost of a copy.

You should take some time to read the GNU GPL. For your convenience, I've reprinted it in Appendix A of this book.