Probably RCS http://www.gnu.org/software/rcs/; It's a fact that it used CVS, then SVN at some point (see https://gcc.gnu.org/viewcvs/gcc/branches/CYGNUS/, a branch created by cvs2svn, a migration tool)

EDIT: Apparently all previous history was lost when they implemented CVS, circa 1997. We could always ask someone in the oldest maintainers file available (https://gcc.gnu.org/viewcvs/gcc/trunk/MAINTAINERS?revision=1...) and have the real answer.