The Rustbolt Resistance is a new faction set to release with the 8.2 patch of World of Warcraft. One of the new zones being added is a place called Mechagon. This zone is reputed to be a lost city of gnomes, ruled by King Mechagon. Though affiliated with the gnomes, the Rustbolt Resistance will be friendly to both Horde and Alliance players.
The idea of having gnomes being friendly to the Horde seems a bit unusual. However, it appears that these gnomes have never been exposed to the politicalization of Horde vs. Alliance and have never needed to pick a side. This differs from the goblins of Booty Bay and other neutral cities who have opted to remain neutral to both Alliance and Horde factions despite the numerous conflicts between the two groups.
Given that the events of the Battle for Dazar’alor left the leader of the gnomes, High Tinker Mekkatorque in a coma, it’ll be curious to see who steps up to lead the gnomes in his absence and what role these new set of gnomes will play.
At present, there is no known information about how to gain reputation with this faction. If Legion was any indication, there will likely be a quest chain that will award a moderate amount of XP as well as world quests. New to Battle for Azeroth is gaining XP via missions from your war table. It is unknown whether new missions will be added for the Rustbolt Resistance.
If Battle for Azeroth follows the same pattern that Legion did, it is possible that the Rustbolt Resistance will be required for unlocking flying across Battle for Azeroth.
At present, there is no faction page built for the Rustbolt Resistance. This is because this faction only exists on the PTR at present and is highly subject to change. Once things become a bit more stable, that page will be added.