Having recently migrated from P3D to X-Plane I struggled with the GPS / Glide slope as well. It took me a while to realise that the operation of the Autopilot / GPS varies from aircraft to aircraft and that on some the Flight Director has to be engaged before Nav or GS etc. I have to activate the AP TWICE to get the GPS to engage when using Saitek Panels
The separate manual for the C172 AP (included in X-Plane docs) is worth the read to make sense of its operation.
Gordon