A330? There's a maximum altitude chart on FCOM 2.04.25 P3, which gives, for a flight with gear extended, the maximum altitude "In order to ensure that there is at any moment of the flight a minimum range of 20 kts between VLS and the maximum speed (limited by thrust or VMO/MMO), the aircraft must remain below the following profile."
21,000 ft corresponds to 210t. So that's likely chosen as a safe assumption, if you took off at MTOW and started climbing with the gear down.