`gcc` is basically a function/package derivation. What it evaluates to depends on the given channel/nixpkgs source you use - in the derivation (something like development/../gcc/default.nix) you will find the exact commit/version that will be used. For packages that routinely use multiple versions something like python2 and python3 are both available but the whole point of nix is to let go of this notion of versioning — you can reference out of tree packages as well, hell you can even use Y compiled with gcc and clang and reference them both.

So your remark that nix is lacking in dependency management is quite far from the truth - it is the only proper solution to the dependency hell problem I’ve seen (of course as well as guix which employs the same model)