Later Macs did output composite sync, so an adapter for one will work with the IIgs too, the biggest difference being the output scan rate. The Multisync 1 and II (I own the latter) were swiss army monitors, they would take H+V, composite, or sync on green without a problem! Apple was pretty loose with the DB-15 video connector in the early days, but seems to have standardized it for the most part with the 040 machines onwards.