GK stands for GateKeeper and represents the Device Access part of PAM.
PA stands for Password Authority and presents the Credential Management part of PAM.
Both were standalone products that were combined to create PAM many years ago.
A1: Correct.
A2: GK devices are access devices, see above.
A3: EndPoints registering with PAM are Server Control Devices.
A4: See A3. Additionally, if the device didn't exist in PAM yet, it will be created and in addition to being a Server Control Devices it should have the default types as specified on the Settings > Global Settings > Basic Settings page.