It seems the current HEAD still can't be bootstrapped using 4.9.3 release. The Docker container (docker run -it schemers/gambit:head) is now built with the following commands to first try release gsc, and if that fails, bootstrap from scratch:
ln -s /usr/local/bin/gsc gsc-boot make || (rm -rf boot/ gsc-boot && make)
That approach would seem to work fine, and saves a lot of time in case the build works with the release compiler (Docker Hub builds are really slow - you get what you pay for...)