Have you tried something like the following: For this example, assume that the gear is on X+2 and that to operate your gear via the sequencer requires a mix such as GER->X+2.
Add a normal mix of RKn -> X+2 (or gear / brake control channel) with a rate of say 30%, 0% and Offset: 0 activated by your Gear switch being in the gear down position.

With the right knob set to brakes off, the gear channel should operate normally and you should be able to adjust the knob to over drive the gear channel and activate your brakes. Depending on the polarity of your gear channel and how much you need to overdrive things you may need to change the rate from 30%,0% to 0%,30% or perhaps even use negate rate values but by looking at the servo monitor it should be easy to sort out from there. Hopefully this helps.

If it could cause problems to have the sequencer cycle the gear if you had left the brake knob active you MAY be able to make the gear sequencer mix activated by a Combo Switch so that the gear will only extend if the gear switch is in the down position AND the brake control is off. If some of this doesn't make sense it may be because I've never seen a setup like you are describing. I'm just trying to offer what I know about DX18 mixes to help solve the problem to the best of my understanding.