Can GNU and other Free and open source tools be used to produce proprietary applications? The answer is definitely yes, although with some restrictions. The best way to learn about these restrictions is to simply read the licenses supplied with the tools and libraries you intend to use. You should do this anyway, and when you do, you’ll probably be surprised to find out what you’ve been agreeing to!

This paper explains the terms and restrictions of the GNU Public License (GPL), the Lesser GNU Public License (LGPL), the BSD license, and the complete GNU GPL.