Hello mmowned,
First of all, please excuse me for my english
Well, I found a good spot where you can farm your Gadgetzan and other goblins factions "very" fast...
There's a total war between goblins and pirates in Tanaris on this place :
(My game is in french but it's not very important to find the spot :P)
All the mobs here give 5 rep for Gadgetzan and 3 rep for the other factions. I earned something like 300 rep in just a few minutes
A little part of the spot (I already killed a lot of mobs when I took the screenshot) :
Note that you can't kill the goblins who are fighting the pirates, so you won't lose rep