Microsoft has said that SharePoint 2013 workflows will be available in Q2 of 2015. Have any dates be set for Nintex Workflows?
I would think that this would slightly depend on whether the Government Cloud instance obtained a CON for Nintex and licensing for it. I know that doesn't answer your question, but may help point you in the direction of who may have the right answer.
Regarding something that specific and slightly not so public information, I would think that it would be more of a government IT/IA thing to get done. I only say that because in some cases such as NIPR/SIPR platforms, Nintex is already available.
Workflows are currently available in the G-Cloud.
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