Police say the man then demanded that they hand over the keys to another car which he drove away in.
Shortly after, the car was involved in a crash at Kemble railway station, where police then believe that the man attempted to steal other vehicles.
A Gloucestershire police spokesman said: "Police were called to the Tesco Extra store in Cricklade Road, Cirencester at 3.40pm today following a report that a vehicle had been stolen from the car wash area at gunpoint.
"The car was subsequently involved in a collision at Kemble railway station and it is believed that the man then attempted to steal other vehicles.
"At 4.15pm officers arrested a 21-year-old man who is currently in police custody.
"No-one was injured at the car wash.
"The weapon has been recovered and is believed to be an air rifle.
"The investigation is at a very early stage and no further information is available at this time."
Anyone who witnesses any part of this incident and has information which might help officers with the investigation is asked to call Gloucestershire Constabulary on 101 quoting incident 396 of July 2.