i don't think they need to make any more models for them. there are a couple of blisters of different figs out there, and models for the special characters are also provided for by GW. if you wanted a GG army, you could easily make one.
I've seen them done... The Gaunt's Ghost armies tend to be pretty small, but look quite good.
Even if GW did decide to do an official release for the Ghosts as an army, they'd be released in metal, as the demand for such an army would not be strong enough to warrant the extra expense that plastics generate.
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