I would think that you'd have a bit of trouble maintaining altitude due to drag at typical cruise alt with the gear down!
Also, if the max retraction speed was 230kts, let's say, then you might also find yourself very near the low speed buffet before you could get the gear back up, although the drag penalty would get you back there much quicker than you'd think!
That's assuming the gear is operating normally. If it is locked down due to unserviceability, then you'd look at the gear down cruise tables in the ops manual to see how high you can climb.