I honestly don't find GG's useless, and never have.
In fact, the ability to grab land with them is among the best abilities in the game, especially once Great Artists could no longer steal land. Having even 5 or 6 GG's is a great thing, as you usually only need 1 to 3 of them to cover units that are actually attacking or defending, and any beyond that can be used to steal land or quickly connect strategic resources. Moreover, the AI is very spotty about pillaging Citadels, sometimes waiting to pillage one until 4 or 5 units have ended their turn on it for several turns in a row.
So long as you are willing to not improve the yield of the tile you put a Citadel on, Citadels and land-grabbing are great. Again, land-grab alone can help you move luxuries and strategic resources from AI lands to yours (or even just extend your border much more quickly over empty ground that contains such vital stuff).
But all that said, the one thing I can safely say is that GGs are the Great Person I buy with faith least-often, since it's maybe only twice per game that I need to land-grab, and usually I have at least one GG to spare that isn't needed for combat.