I think there may be some confusion on the S2 designation. I believe you may be referring to Security 2, S2, which is related to Z-wave security products.
Alarm sensors for any Alarm.com compatible panel do not use Z-wave for communication and “S2” would not apply. Z-wave sensors are not compatible with the alarm panels.
Z-wave on the alarm panel is used for automation devices, primarily lights, locks, and thermostats.
Alarm sensors utilize manufacturer RF frequencies for communication.
Regarding the GC3, they use 345 Mhz Honeywell/2GIG sensors. The panel has jamming detection for 345 mhz RF, but these sensors do not have encryption.
Qolsys S-Line sensors are encrypted 319.5 mhz sensors.
Spoofing non-encrypted sensor signals is a possibility with a dedicated user in proximity of the signals who has the time and the right set of tools. (If I recall, that is what is being done in the linked thread)
Firmware updates since then have included improvements, but I’m not sure of directed follow ups from the panel manufacturers named. We can check with 2GIG and follow up here.
If encryption is a concern in setting up, you will want to look into Qolsys S-Line sensors, as they have encrypted communications.