How to get the Civilian P-51 Autopilot perform RNAV Approach

What I don't understand, is how do I get the Autopilot to drive the HSI and fly the Civilian P-51 down the RNAV GPS Glide Path?

I have the rotary switch set to NAV, and the Garmin Flt1 GTN 650 set to GPS, but the HSI does not track the horizontal. However, it does show the vertical path deviation with a carat?

Either this plane has to be flown manually down the vertical and horizontal path, or I am doing something wrong, because I am expecting the autopilot to take control and automatically fly the plane down the glide path?

Does the Civ P-51 handle WAAS approaches, in addition to standard RNAV approaches?

I will try out the ILS capability later.

Bill Clark