At the base edge of the GTN 750 there are some selectable icons. Ensure that the CDI icon for GPS is selected. It should read GPS in the window. The alternative selection is VOR.

Also, endure "unsuspend" is selected. You will see the curved magenta line appear for the LNav and provided the aircraft's autopilot controls are set to receive the input from the GTN, the aircraft should follow the path.

The aircraft's VOR/HDI receiver should be working with the GTN. But in the L1011, it's the HDG bug that gets controlled by the GTN. I have no control over the HDG bug during GTN LNav. When HDG bug is required, I de select GTN GPS, going to VOR and briefly disengage the autopilot. Then re engage the autopilot and select HDG.

Hope this helps. :)

Regards
David