Copy pasting from the nix manpage:

keep-derivations ⟨#conf-keep-derivations⟩

          If  true  (default), the garbage collector will keep the derivations from which non-garbage store paths were built. If false,
          they will be deleted unless explicitly registered as a root (or reachable from other roots).

          Keeping derivation around is useful for querying and traceability (e.g., it allows you to ask with what dependencies  or  op‐
          tions  a  store  path was built), so by default this option is on. Turn it off to save a bit of disk space (or a lot if keep-
          outputs is also turned on).

Since we're gc'ing outputs and it specifies "will keep the derivations from which non-garbage store paths were built", I believe this will also gc derivations?