Well I guess it depends on your map type. If on a pangea, GPs are invaluable bc you'll definitely be competing for the favor of cities with other religions. On a continets map, there will likely be "neutral" continents with no native religion, where the key priority is not so much removing other religions, but quickly establishing followers of yours to exert pressure and get a lead. In those situations, missionaries may be more beneficial bc of the lower cost and increased movement.
It's highly situational; especially depending on what you're trying to get out of the religion system. If you need to maximize the number of adherent cities to get your bonuses, GPs are it. If you're just trying to maximize followers (Like if I have tithe, for example), then GPs aren't as invaluable. All depends on your objectives. GPs are, however, more versatile and valuable in general. I'd say once it starts getting to about 6:1 or so, maybe start looking at missionaries (Although their cost going-up over time neutralizes this).