"maximum altitude at which L/G can be extended" is 25,000 ft. So there is no altitude limitation for flying with gear extended nor to retract.
Also, L/G operating has only KIAS limitations, not Mach.
So, theoretically, you can climb with L/G down to whatever altitude provided you do not exceed 280/.67, and even retract it provided KIAS is below 220.
theoretically