Nix will interpret that as gymnasium <array containing the variable classic-control>.
I’m surprised this evaluates, frankly, you should get something like “not a derivation” or “missing variable”.
Nixpkgs doesn’t support this optional dependency shortcut syntax directly, and the package doesn’t add any additional support either. You would need to manually install every package that gymnasium refers to. Alternatively, you could use uv2nix to resolve it instead.