The incident is reported to have taken place in Garrick Street at around 3pm on Saturday 20 March 2021.
The suspect, who was sitting on a bench with a group of men, is described as being white, in his 50s and around 5ft 10ins tall.
At the time he was wearing a black leather jacket, a striped shirt, black trousers, black boots and a green and black scarf.
A 56-year-old man from Gravesend has previously been arrested on suspicion of the offence and remains under investigation.
Police Constable Ben Slocombe, the investigating officer for the case, said: ‘We have reviewed various lines of enquiry, including checking for CCTV and interviewing a suspect.
‘We do however believe there are several witnesses who have important information and are yet to get in contact with us.
‘This includes the men who were sat with the man, nearby shoppers and some passengers who got off a bus that stopped nearby.
‘If any of them have information that they believe can assist us, I would encourage them to contact us at their earliest convenience.’
Anyone with information is asked to call Kent Police on 01474 366149 quoting 46/46187/21.
You can also contact the independent charity Crimestoppers anonymously by calling 0800 555111 or using the anonymous online form available via this link: https://crimestoppers-uk.org