I think Gallade is better. Both secondary typings help with Dark types, but Gallade’s STAB options won’t help with Ghost types. But unlike Gardevoir, you can get a full set with Gallade once you evolve it. Gardevoir gets Moonblast at level 49 and Psychic at 42, but Aqua cutter, Leaf blade, Night slash, and Sacred Sword are learned at Level one(Psycho cut won’t be available until level 42, but it won’t matter as much as Gardevoir as all the other moves will get you there) You can also gat a Dawn Stone in the East and West Providence 3 area, meaning that you can get Gallade as soon as Level 20. Also, you will be using more offense in a nuzlocke so the Fighting type is more useful. It is your choice, but I would recommend Gallade.
Dawn stone source
