Air carrier Captain reported GPS interference during push back. Crew was assisted by company on a work around and flight departed with no further anomalies.

Narrative

Per an ops alert; says should consider DEL as an area of known or suspected GPS interference. Therefore; during preflight; refer to and follow the Flight Manual GPS Interference annunciated checklist. After complying with the preflight portion of the checklist to include turning off the GPS; we had no anomalies or messages while at the gate. It was not until we started our pushback during taxi did the ANP begin to drift and; within several minutes; drifted beyond the RNP causing the NAV UNABLE RNP EICAS message to appear. Since this is listed as a GPS-related EICAS message in the Flight Manual supplementary checklist; per the ops alert; it says DO NOT TAKE OFF. We contacted Dispatch and got Maintenance Control and Chief Pilot on the call. Maintenance Control deferred to Chief Pilot. The Chief Pilot contacted the fleet manager. He informed us there is a fix and stepped us through the process which was not included in either the Ops Alert or the Flight Manual. From a stopped position; on POS INIT Page 1/4; select LAST POS at R1 and drop to scratch pad and paste at SET INERTIAL POS at R5 (dashes should be shown). This procedure cleared the EICAS message. Dispatch; Maintenance Control; Chief Pilot and the fleet manager agreed with the Crew that we were good for departure now. The fleet manager did mention that selecting the GPS on should clear it too if no obvious jamming (which was our case); though not consistent with Flight Manual supplementary checklist.Unfortunately; the LAST POS fix was only temporary and by the time we got back in sequence for departure; the ANP once again drifted to exceed the RNP triggering the EICAS NAV UNABLE RNP. So we went back to POS INIT page 1; but there were no dashes at R5 to insert the LAST POS again. So we proceeded with the other method by temporarily turning back on the GPS. This remedied the drift and cleared the EICAS message. We returned the GPS to off again per the Flight Manual supplementary checklist for departure. On departure; we had no anomalies.