Hey there, I could well be doing something wrong here but with the GTN750 in the SF260, the behaviour of the CDI needle on the HSI is not what I expect. In most cases, the CDI shows no deviation from the GPS track even when the 750 is clearly showing one. Toggling the autopilot into NAV mode will usually 'refresh' the HSI and the CDI will jump to the expected position.

SF260 version 4.4.6
Not quite sure exact version of RXP GTN 750 but downloaded it only a couple of days ago
X-Plane 11.50b12.

Example image of CDI not reflecting course deviation:

 

Example of CDI needle showing correct course deviation once AP NAV mode engaged:

 

My GTN settings:

Ah - might have fixed this... anyone reading in future try checking your GPS source is set to GPS1 in the RXP settings: