Thames Valley Police have issued an urgent appeal for information about Gordon, 53, an Oxford man whose whereabouts remain unknown in London.
Gordon was reported missing at 1.45pm on November 5 after failing to return to his home in Wheatley, Oxford, police said.
Officers believe he may have travelled from Thornhill Park & Ride on an Oxford Tube bus to the Kensington area of London.
When he was last observed, he had on a black raincoat from North Face, dark blue jeans, and Nike trainers in black and white.
He also wears glasses, has green eyes and a white beard, and is described as a white male of 5ft 5in with a heavy build and thinning, shaven white hair.
Police have said they are concerned about his wellbeing and have asked anyone with information to call 999 and quote reference number 43260579455.