B777-200
Flight Crew Air Transport Pilot (ATP)
Confusion
Deviation / Discrepancy - Procedural Clearance; Ground Incursion Taxiway
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented
Chart Or Publication; Weather; Human Factors
After cat 3 approach and landing on Runway 07R; we exited on Taxiway R and expected to taxi North on R to the ramp. Ground instructed us to taxi on G and G1 to Mike and then left on Mike to R. The visibility was worse north of the runways and I was taxiing slowly but still unable to see very far ahead. The Commercial II chart seems to show G1 stopping at L and has a break between L and M. Because of the visibility and darkness we couldn't see G1 continuing Northward and turned left on L to reach R instead of continuing north to reach M and turning left there to reach R. Ground saw us turn left on L and said that  was OK and for us to continue to R and turn North on R to the ramp. We should have stopped and confirmed with ground what the taxi instructions were. The Commercial II chart is unclear and we should have confirmed with ground that G1 continued north even though it is not shown on the chart. I should have been more wary because of the limited visibility.
B777 reports not complying with taxi instructions after landing at PANC in low visibility. A possible error in the Commercial II terminal taxiway diagram is brought to light.