I am behind a firewall and using gitproxy:
exec socat - socks4:proxy-server-name:$1:$2
$ git clone git://gcc.gnu.org/git/gcc.git
Cloning into 'gcc'...
fetch-pack: unexpected disconnect while reading sideband packet
fatal: early EOF
fatal: fetch-pack: invalid index-pack output
$ git clone --depth 1 git://gcc.gnu.org/git/gcc.git
$ git clone https://gcc.gnu.org/git/gcc.git
I have no problem on Fedora 34.
This is almost surely going to have to end up on the git list... but you might try to use:
git -c protocol.version=1 clone git://gcc.gnu.org/git/gcc.git
to see if that has an effect.
The release notes mention a fix to fetching which affects the v2 protocol:
Either way, I would suggest bringing this up on the git list as that's where the folks most likely to know the problem will be found.
$ git -c protocol.version=1 clone git://gcc.gnu.org/git/gcc.git
git 2.33.0 fails and git 2.32.0 works.
"socat -t 10" also works.