Air carrier Captain reported GPS jamming and spoofing resulting in system unreliability and GPWS pull up commands during cruise. ATC was informed and flight continued.

Narrative

Just prior to position RIVOS we experienced GPS jamming; first indicated by TRANSPONDER L EICAS message. I immediately selected GPS updating to off and called for the TRANSPONDER L and GPS Data Unreliable checklists. We were subsequently spoofed; inducing a GPWS PULL UP command. Given the CAVU conditions and altitude I selected the TERRAIN OVRD switch. The spoofed position was over the Black Sea and UTC +4 minutes. I also contacted Dispatch regarding RNP 4 airway routing through the PRC (Pilot Reference Card) starting at position KAMUD; on W186 and Y1. Dispatch advised that notify ATC for relief from RNP 4 requirements. We also discussed alternates in the vicinity in the event relief was not given. ATC was informed prior to FIR entry and seemed to reply that this was OK.Cause: It appears that either a lack of contingency planning is being performed for the possibility of GPS issues along this route; or that this route simply gives no contingency options in the event of a GPS jamming/spoofing event occurs. Given the frequency of GPS jamming/spoofing events along this route; a viable contingency plan that doesn't hinge on ATC relief should be a requirement for flights transiting from Location X to Asia.Suggestions: Given the frequency of GPS jamming/spoofing in the area; select routings that do not require GPS; or plan other means of contingency when operating into Western China from Location X.