Air carrier B777 Captain reported GPS jamming in-flight. Although well above any terrain; the pilot received a terrain warning and earlier; a GPS jamming message.

Narrative

Departed VIDP enroute to ZZZ; as Aircraft X. Approximately 30 minutes after departure and level at FL300 just northwest of the SULOM fix and roughly 250 miles from the VIDP airport; we received a GPWS TERRAIN TERRAIN" warning (both aural and visual). We executed the GPWS Actions and Callouts per the B777 AOM. The warning continued up to the aircraft's Max Altitude of FL325. At that point; we confirmed both on our own and with ATC that we were on course and above all terrain. The GPWS warning continued to display and sound until we manually switched it off. After cycling the TERRAIN OVRD switch; the system appeared to reset itself and operated normally. It should be noted that the GPWS warning was proceeded by a GPS jamming display received earlier in the flight; GPS Jamming warning. With GPS jamming occurring on a regular frequency on flights in the Middle East; I believe that a thorough review of jamming affects on the different aircraft systems during recurrent ground school training would be very advantageous to those who fly these routes."