Unless you're using them for competitive play, it ultimately depends more on which aesthetic you enjoy, as you can clear the main storyline with either of them on your team. Gallade is an ATK / physical sweeper while Gardevoir is moreso used as a SP.ATK / special sweeper. Gallade has a useful typing that makes it non-vulnerable to Dark moves which is quite common in open-world gameplay, especially against the Team Aqua and Magma teams.
I'm biased towards Gardevoir in Hoenn because one carried my entire team when my dumb five year old self released my starter ;'>
You can buy a dawnstone on the GTS, and you can also find them on wild Ralts'. In the future, you can check the method of obtaining any item in the game here.
Good luck on your nuzlocke run!