From Rosetta Code

GCL is Gnu Common Lisp.

GCL is a Common Lisp currently compliant with the CLtL1 standard. Lisp compilation produces native code through the intermediary of the system's C compiler, from which GCL derives efficient performance and facile portability. Currently uses TCL/Tk as GUI. (from http://savannah.gnu.org/projects/gcl)

See Also

Wikipedia:GNU Common Lisp