The only concern with adding multiple roles to any server is the load placed on the server. However, both TS Gateway and VPN access are neither CPU nor RAM/HDD intensive, so this would not be an issue. The only potential problem would be saturation of the Network Card, but that again would not be a problem if users are connecting from an external source.
Network Policy Server (NPS) would need to be configured with the appropriate Connection Authorization Policies and so on to allow the remote connections through either TS Gateway or RRAS. This is simple though, and most of it is taken care of during the installation of those 2 roles.
In short, no, it won't be an issue.
-Matt
