People get all manner of different things out of ABDL. I personally lean way more on the kinkier/BDSM side of things.. which for the most part if who these products are aimed at.
My general experience has been that no, most don't really make effective gags just due to the shape of a pacifier in relation to the anatomy of ones head. You either need a really big pacifier (which ends up being uncomfortable for any real long term wear) or a very elaborate (and tight) harness that loops around you chin, neck, cheek, and over your head.
That said, I do love the idea of being forced to use a pacifier, and despite pacifier gags not really being that effective, like many things ones head-space can make up for limitations of the real world.