Aside from Michael Tiemann, who worked out the front end for GNU C++, and Richard Stallman, who worked out the back end, the following people (not including those who have made their contributions to GNU CC) should not go unmentioned.
Doug Lea contributed most otherwise unattributed classes.
Per Bothner contributed the iostream I/O classes.
Dirk Grunwald contributed the Random number generation classes, and PairingHeaps.
Kurt Baudendistel contributed Fixed precision reals.
Doug Schmidt contributed ordered hash tables, a perfect hash function generator, and several other utilities.
Marc Shapiro contributed the ideas and preliminary code for Plexes.
Eric Newton contributed the curses window classes.
Some of the I/O code is derived from BSD 4.4, and was developed by the University of California, Berkeley.
The code for converting accurately between floating point numbers and their string representations was written by David M. Gay of AT&T.