We have published a white paper that analyzes RD Gateway capacity planning in Windows Server 2012.
In Windows Server 2012, Remote Desktop Gateway (RD Gateway) enables authorized remote users to connect to Remote Desktop Protocol (RDP) accessible resources on the corporate network, from any Internet-connected RDP client that supports RD Gateway (such as the Remote Desktop Client that ships with Windows).
This whitepaper contains scalability results, testing methodologies, analysis, and guidelines for RD Gateway. It describes the most relevant factors that influence the capacity of a given deployment, methodologies to evaluate capacity for specific deployments, and a set of experimental results for different combinations of usage scenarios and hardware configurations.
For more information about Remote Desktop Services, see the Remote Desktop Services TechCenter.