IIRC, the Galleases (that's a tough word) were arrayed as a kind of flying wedge at the fore of a larger fleet of conventional galleys. They used their mass & cannons to plow into the Turkish fleet, disrupting their formation. Individual galleys need maneuvering room. If you pack them together you can destabilize a whole group of galleys and make them vulnerable to capsizing or boarding.
Whatever the reasons for their success, the Galleas was a 'silver bullet' weapon at Lepanto. The Turks simply had no weapon or tactic to counter it.