Lincolnshire Police is appealing for information about the theft, which occurred at the Betfred on Silver Street at around 8.20pm last night (Wednesday).
The man fled the scene on a small red/orange chopper style bike in the direction of Caskgate Street and Bridge Street.
Police said no weapons were used in the robbery and no-one was injured.
The man is believed to be over 6ft tall and was wearing a bomber jacket with blue faded jeans.
Anyone who witnessed the man leaving the scene or has information about his man’s identity is urged to contact police on 101.
Information should be given for the attention of DC Haddock of Gainsborough CID. Alternatively people can call Crimestoppers anonymously on 0800 555111.