Your gnome will need to be exalted with Gnomeregan in order to get the heritage armor. But only your main needs to have completed the BFA rep stuff.
I’ve found the easiest route is to go to Northrend and do the ToC dungeon with a tabard. Just completed it on my gnome last night. You get roughly a little more than 1k points for each run through the dungeon. It is pretty quick. Although you’ll run into the to many instances wall because of how fast you can complete the dungeon.
See this blue post on a similar question about tauren.