A couple of my dogs (Rosie and Cosmo) responded very well to the Gentle Leader but it has absolutely no affect on Otis the Lab/Golden cross, he is just too strong. He gets a body harness with the leash attachment on his back and sometimes a secondary attachment on his collar. Otis will also wear the harness for days if he has to but won't tolerate any down time in the Gentle Leader.

Whichever is used, fitting is key, and dogs change weight often enough (usually seasonally here) for the harness to require readjustment. Harnesses can loosen in use too. Or maybe I'm just fussy.