Yes, you can filter the store pages based on panel compatibility for the GC3. Here is a link to the filtered store for sensors compatible with that panel:
In addition, will my older Alarem.com cameras work with Surety subscription?
With a plan that includes video support you can use the camera models listed here:
If I were to get a whole new panel, will my existing door and window sensors, motion sensors and smoke detectors work with it?
That depends on the model of panel. The panel would need to support 345Mhz sensors. A 345Mhz version of the IQ Panel 4 is found here: