The thieves forced the security grills at the saleroom at around 10pm on February 6, the night before their most recent sale. They then entered the building and broke into two locked cabinets, taking mostly Chinese jade, works of art and silver.
It is believed they may have attended the auction viewing in order to target what items to steal.
The combined estimated value of the objects stolen was around £30,000 and among them was a rare Ming Dynasty greenish-white jade finial which had been estimated at £2000-3000 for the sale.
Anyone with information should contact either Dominic Winter on 01285 860006 or Gloucestershire Police on 101 (quoting incident no. 453).