Here is a bit more information taken from that page I linked that should apply to you as you seem to have unlocked Mechagon on your Horde but not Alliance.
If you have done these achievements on any Alliance character(s), you can head to the Alliance Embassy in Stormwind on that character and start the unlock questchain with Aysa Cloudsinger and accepting the quest “Urgent Care”.
If you have done these achievements on a Horde character, you will then need to login on a level 120 Alliance character.
If this character has already completed the Mechagon Island intro questline that ends with the quest “Welcome To The Resistance”, then the character can head to the Alliance Embassy in Stormwind and pick up the quest “Urgent Care” from Aysa Cloudsinger to start the Allied Race unlock questline.
If this character has not unlocked Mechagon Island, you can head to the Alliance Embassy in Stormwind and Gila Crosswires will have the quest “Fame Waits For Gnome One” which will lead the character through the Mechagon Island unlock process which then leads to the mechagnome unlock questline. If you are in this situation, you will not need to go to Nazjatar.
Note: 120 is the presquish level. You’ll need a level 50 Alliance character.