mgb74, we share a common language so there's no need to reuse perfectly good words in surprising ways.
What you call a GG back is usually called a focusing panel. Graflok backs' focusing panels are removable, spring backs' focusing panels aren't removable, are pulled back to allow insertion of a film holder. Your Century has an integral Graflok back; you may not be familiar with spring backs.
"Graflex type backs" is a terrible neologism, in part due to Graflex Inc's (and sucessors and predecessor's) extremely confusing nomenclature. A Graflex back, strictly speaking, was standard on Graflex SLRs; a Graflex back has a fixed bar on one side of the gate and a sliding bar on the other, does not have a focusing panel.
What you seem to be thinking of is a roll holder made by Graflex Inc. (and successors). A roll holder isn't a back. It attaches to a back. Most of the rollholders that Graflex Inc. made attach to Graflok backs; the focusing panel is removed and the Graflok back's sliders hold it in place. Some, however, attach only to Graflex backs. These are marked Graflex 23. Early Graflex 23 roll holders will also attach properly to Graphic backs, later ones won't. I have both flavors.
Similar roll holders that attach to Graflokish backs are made by a number of makers, Horseman for one. I've never tried a Horseman roll holder on my Graphics, have it on good authority that the backs' edges are too thick for them to be used on Graphics.
About film flatness with roll holders that Graflex made. Some users report film flatness issues with older ones whose shells lack the pin rollers that are in newer shells but hot with the newer ones. Other users report film flatness issues with all vintages. And others still report no film flatness issues with any of them. The new type shells are marked RH- (-8 for 6x9, -10 for 6x7, -12 for 6x6), were delivered with film carriages that have advance levers. Older types, marked Graphic 23, were delivered with film carriages that have film advance knobs. There are some intermediate shells, marked Graphic 23, not RH-8, that have the pin rollers. These sit at the ends of the gate; shells with pin rollers have gates ~ 3 mm shorter than shells without. SKGrimes will add pin rollers to a Graflex roll holder's shell.
You're right about the risk that a carriage for one format can turn up in a shell with a gate for another format. These bastards can be recognized by comparing the largest number on the frame counter dial (8, 10, 12) with the gate.
You didn't mention that when Graflex folded the roll holder tooling was sold to a company called SubSea. I have a SubSea RH-8. Its frame counter goes to 8 but the counter gearing was for an RH-10 when I bought it. Armatos put the right gears in for me.