Linux is a Unix-like free operating system kernel developed initially by Linus Torvalds. Nowadays this kernel is maintained by thousands of developers, many of which code it in their free time as a hobby.
Many more usefull information can be found at Wikipedia's Linux article.
Linux's official kernel versions can be found at http://www.kernel.org
NOTE: aMule is a separate project and is not realated to GTK in any way.