If you go to the fuel penalty factors table for all gears extended it refers to PRO SPO 25-10 which is Flight with gear down. There it says that for a failure of the retraction system (whether mechanical or cerebral - my entry) then the limits of that section apply. At FL240 fuel flow from the planning tables appears to be in the order of 1600-1800kg/hr per side depending on weight. This with an indicated speed of 210 for a TAS of 302.