Clapham, which also owns The Real Greek chain, has received a takeover approach from an unnamed suitor and talks over a possible deal are ongoing.
But chairman David Page did not gloss over the problems facing the company.
He said: “The UK trading environment and outlook remain challenging and although August has shown signs of a return to the improved pre-World Cup sales trends, performance remains volatile.”
The company recently reported a 50 per cent rise in profits in the year to 28 March.
However, the World Cup and glorious summer weather contributed to a slump in the number of restaurant-goers.
Clapham operates 53 GBK restaurants in the UK and 15 internationally, as well as six restaurants under The Real Greek brand.
The stock closed down 5.2 per cent at 68p after the update, which came before the company’s annual general meeting. The firm’s market value is £28m.
Last week Middle Eastern giant Landmark Group agreed to pay £90.3m for Italian restaurant chain Carluccio’s.
Page added that the board was continuing talks with the possible suitor but no offer was certain.