For future reference, you can also do this for most files:
$ nix-locate --top-level libstdc++.so.6 | grep gcc
libgccjit.out 0 s /nix/store/76vxcz4qm3v22li7dxcvpyn4hl4ivnki-libgccjit-10.3.0/lib/libstdc++.so.6
libgccjit.out 1,903,088 x /nix/store/76vxcz4qm3v22li7dxcvpyn4hl4ivnki-libgccjit-10.3.0/lib/libstdc++.so.6.0.28
libgccjit.out 2,498 r /nix/store/76vxcz4qm3v22li7dxcvpyn4hl4ivnki-libgccjit-10.3.0/lib/libstdc++.so.6.0.28-gdb.py
gcc-unwrapped.lib 0 s /nix/store/x17b1wq871r4ycrxyy1n85ja09dxq3ih-gcc-10.3.0-lib/lib/libstdc++.so.6
gcc-unwrapped.lib 1,903,088 x /nix/store/x17b1wq871r4ycrxyy1n85ja09dxq3ih-gcc-10.3.0-lib/lib/libstdc++.so.6.0.28
gcc-unwrapped.lib 2,494 r /nix/store/x17b1wq871r4ycrxyy1n85ja09dxq3ih-gcc-10.3.0-lib/lib/libstdc++.so.6.0.28-gdb.py
nix-locate comes from the nix-index package, which will index your nix store, so if you already have the package installed, you can inspect which attr produced it.