Modern motherboards come with a built-in LAN controller.
What happens when I am using a GPRS modem - does it involves the usage of that LAN controller in any way?
Depends on the GPRS Modem in question!
If it is a GPRS card modem for a laptop (Express Card/similar) or desktop(PCI Express), the answer will be no.
If it is a GPRS Modem that has only a network card port, the answer will obviously be yes.