Sure. You use a relay with an extra pole as a latch, and use another one controlled by Mach to break the latch.
You need two relays and a separate power source (like a little 5V wall wart).
One relay is the one that powers the gecko and the pump. It needs an extra 'pole'. Call this the "power" relay.
The other is a relay controlled by Mach3 (from a control line on the gecko). Call this the "control" relay.
Connect one side of the power relay coil to the separate power supply +.
Connect the other side of the power relay coil to the NC connection of the control relay.
Connect the common connection of the control relay to the NO connection of the momentary switch AND the NO connection of the extra pole of the power relay.
Connect the common connection of the momentary switch to ground.
Connect the common connection of the extra pole of the power relay to ground.
Connect the - of the extra power supply to ground
When everything is off, both relays are off.
When you depress the momentary switch, the coil of the power relay is energized (one side is connected to + of the extra supply, the other is connected to ground through the NC connection of the control relay, through the switch).
The relay activates, and now the extra pole also connects the coil of the power relay to ground. At this point, the switch can be released, the extra pole is holding the coil energized. This is the latch.
When Mach3 turns on the control relay, the NC connection through the control relay is broken. The power relay turns off, powering off the gecko and the pump.
Mach3 then turns off the control relay. The power relay is not activated, because neither the momentary switch nor the extra pole is on.