No, unfortunately not.  When you engage the GPS module in the 2820, the external serial port is disconnected and routed internally.  This means that no data can be sent from or received by D-RATS because the radio is processing the data stream itself.

You can selectively enable/disable the GPS in the radio as necessary, but you can only use D-RATS when it is in the disabled state.

Note that if you see a G-D icon in the top corner of the screen, the GPS mode is still enabled and D-RATS will not work.