`nix.gc.automatic` is implemented as a systemd timer to run `nix-collect-garbage` periodically.

`nix.optimise` is just a nix conf option. Meaning anytime nix creates a store path, it will automatically dedup files by hardlinking them to a `/nix/store/.links/` path. There's no timer involved with `nix.optimise`. You can manually force existing paths to be "optimised" by doing `nix-store --optimise`