Debian bullseye will be based on a gcc-10 package taken from the gcc-10 upstream branch, and binutils based on a binutils package taken from the 2.35 branch.
I'm planning to make gcc-10 the default after gcc-10 (10.2.0) is available (upstream targets mid July). binutils will be updated before making the GCC switch. The GCC 10 switch involves some minor library transitions for D, gccgo, M2, which should be no-brainers. The gnat transition will be handled separately by the debian Ada maintainers. binutils should be pretty stable until the bullseye release, not planning an update to 2.36. GCC 10 should be updated to 10.3, or close to 10.3 (the release date is not yet known, could be Feb 2021). I'd like to get rid off GCC 8 and GCC 9 for the bullseye release. Matthias