GPL is An acronym for the GNU General Public License, a family of software licenses that are common among FSF developers and many others;
However it is not at all favoured for the Tcl core, as it severely restricts what commercial development may be done with it. IMHO it works better for applications, and though there is the LGPL (Lesser or Library GPL) and other derivative FSF-sponsored licenses to address these sorts of concerns, Tcl's modified Berkeley license is still the currently preferred (and required for Tcl core work) license.