It seems, that you are using your internal RH links in public gitlab repo, well...

$ uname -a

Linux localhost.localdomain 5.11.14-200.fc33.x86_64 #1 SMP Wed Apr 14 15:25:53 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

$ centpkg clone --anonymous -b c9s gcc

Cloning into 'gcc'...

warning: redirecting to https://gitlab.com/redhat/centos-stream/rpms/gcc.git/

remote: Enumerating objects: 181, done.

remote: Counting objects: 100% (181/181), done.

remote: Compressing objects: 100% (94/94), done.

remote: Total 181 (delta 96), reused 155 (delta 81), pack-reused 0

Receiving objects: 100% (181/181), 127.24 KiB | 259.00 KiB/s, done.

Resolving deltas: 100% (96/96), done.

$ cd gcc

$ centpkg sources

Downloading gcc-11.0.1-20210324.tar.xz

Could not execute sources: (6, 'Could not resolve host: sources.stream.rdu2.redhat.com')