Either way, his policy of "hire nuke-nin then have them killed instead of paying them" would only work if it's a new policy instituted when he hired Zabuza or if he's only done this a few times before. Since Zabuza hadn't even suspected a betrayal, this to me suggests it's not a regular occurrence regardless of what Gato claimed in canon.
To add to this, we see in canon Gato showed up personally to the bridge, where he announced his intent to finish off the wounded survivors (including Zabuza) and claim any bounties.
Like ... he deliberately went down there in person, and put himself less than 100ft from the high-tier ninja he was planning to betray and murder.
Maybe Gato is a big deal in financial circles, but he's clearly smalltime in the murder game, because he obviously had no real idea what these people were capable of. He employed a small army of normal thugs, and was probably putting the ninja in a mental box labelled 'tougher than the normal thugs. Like, maybe worth ten of them?'
Because the idiot clearly didn't know he was committing suicide by heading down there personally.
Any past dealings he's had with ninja were probably low level scrubs, unaffiliated to any of the major villages. Like the guys from the satellite villages mentioned earlier in the fic, who aren't either the result of ninja clan eugenics, or just a freak of nature, giving them bullshit levels of chakra most of the population just
isn't capable of. And probably didn't attend Academies, where they were handed a number of starter jutsu.