Gillian, who would now be 53 years old, was last seen at her home in Patcham on August 23, 2017.
She was reported as missing the following day after not returning home from a walk on August 24.
At the time of her disappearance Gillian was described as being 50 years old, white, 5’6″, of medium build and with shoulder-length brown hair.
She was last seen wearing a thin blue or grey 3/4 length sleeve top, a grey ‘stone’ effect Peter Storm waterproof jacket, brown ankle boots, a knitted grey scarf and carrying a small grey Karriemor rucksack.
Senior investigating officer Detective Inspector Dee Wells said: “I cannot begin to imagine how difficult the past four years have been for Gillian’s partner and friends.
“Since she was first reported missing our officers have worked tirelessly alongside partner agencies across the country to find any information on Gillian’s whereabouts but have so far been unable to locate her.
“We urge anyone who has any information which could be relevant to the investigation, no matter how small, to report it to us, either online or by calling 101.”