What packages the man pages for GCC on Debian 10 Buster / Testing?

The manpages are provided in contrib packages, gcc-doc etc. (See the links at the top-right of the linked page for all the releases where the package is available.)

Debian 10’s default compiler is GCC 8. The GCC 8 documentation wasn’t packaged in time for Debian 10’s release, but it is available in backports, along with the corresponding gcc-doc package. To install it, you need to enable backports with contrib and non-free, and install it from there explicitly:

echo deb http://deb.debian.org/debian buster-backports main contrib non-free | sudo tee /etc/apt/sources.list.d/buster-backports.list
sudo apt update
sudo apt install -t buster-backports gcc-doc

The gcc-doc for Buster is not really available yet, but can be installed, with a bit of finagling, from buster-backports. Hopefully this answer will become obsolete soon.... As root:

echo "deb http://deb.debian.org/debian buster-backports main non-free" >/etc/apt/sources.list.d/buster-backports.list
apt-get download gcc-8-doc
dpkg -i gcc-8-doc_*.deb

It won't survive a dist-upgrade though because of the unmet dependency.