I have completed all the requirements to unlock the Mechagnome. I completed them on a Horde character and then logged to do the final bit of the questline on my Alliance character. I’m not seeing the quest giver anywhere. As far as I can tell I’ve completed everything on the alliance side to unlock Mechagon but can not find the quest to go there either.
Did you unlock Nazjatar up to & including A Tempered Blade quest?
The completion of that quest is meant to trigger Fame Waits for Gnome One – which is the breadcrumb for Alliance that leads them to opening up Mechagon island.
If you are unable to get the breadcrumb quest, try checking by the flightmaster in Boralus for Tinkmaster Overspark. He should have the quest, The Legend of Mechagon for you.