The Gun isn't in the Canary Wharf estate, it's on the east side of the Isle of Dogs. Although it's a short distance as the crow flies from the Wharf, getting between the two is not obvious. The walking route that was available in previous years through Wood Wharf has been closed because of construction work.
For anyone in the Canary Wharf area, I'd suggest jump on the DLR to Cutty Sark (which is in the heart of Greenwich) then follow the river path East to The Trafalgar and The Cutty Sark.