I think it has to do with us meeting him in the Broker City of Tazavesh in the Shadowlands. He got access to Ethereal/Broker Tech and relics from other realms and was able to use that to springboard himself from deposed Trade Prince to ‘Overlord’ by using things no other Goblin had seen or had access to, and we know that, no matter what we may think of the Goblin himself, Gallywix is frightfully intelligent and prone to bouts of self-important aggrandizement .
He has dozens of plans percolating in the background, and plenty of twists and backups in place, but he’s also hamstrung by his need to prove that Gallywix is the Main Man, that he’s the richest, the smartest, the most powerful Goblin that was, is and ever will be on the face of Azeroth, and that need for adulation and recognition cripples his plans.
Give his casino in Undermine and his connection to a certain Primal Power, I suspect that, whatever happens in the Raid itself, Gallywix used his lingering respect as the former Trade Prince of the Bilgewater Cartel and his remaining stockpiles of wealth, kajamite and influence to bring the other Cartels in for games of chance, with his loot from the Shadowlands giving him a kind of unseen, subtle leverage in these games that, when coupled with his own intelligence and the usual Goblin shenanigans with such places, the ability to cheat at the games and never be caught.
With wealth lifted from the other Trade Barons and Princes in flawless games of chance and wit not only swelling his coffers but making him seem more competent and skilled than his former counterparts, that would convince many lower-ranking Goblins to switch sides, especially if Gallywix ‘magnanimously’ offered to waive the gold he was owed in exchange for favours from the other Trade Princes.
Kezan might have been, and still is, sole property of the Bilgewater Cartel, but Undermine has always been neutral territory for all of Goblinity, and a great many Goblins probably were eager to point and jeer at Gallywix’s fall from Trade Prince to lowly gambling hall owner, but as his operation grew, his coffers groaning beneath the weight of their new macaroons, his buildings growing larger and more ornate with every month, his leg-breakers swelling in number and boasting exotic tech well beyond what the average mook could be equipped with and his influence re-grew at astonishing speed, the society and cultural ‘norms’ of Goblins would have pushed many desperate or ambitious Goblins to sign on with Gallywix to better their own lives.
After all, if you’re not with Gallywix, you’re a target, either for extortion and exploitation, or a actual target for his goons to remove from the board.