Controlling LinuxCNC and the gui while switching a gear

I want to switch the gears on a Deckel FP3A CNC, when a g-code command like "S10000" or the user 
by the gui wants to change the speed of the spindle.
My problem is not changing the gear self, but i didn't know how to do the following points:
- Suspending the actual g-codo command until the switching of the gear was done.
- Disabling the controls of the gui for the same time.
Is there any way to do this?
 

M66 wait for input will wait for a digital input pin to go true.
Ideally you would trigger the toolchange and when its completed set a digital i/o pin that is being waited on to be  true.

Thank you for your answer and you are right, with using "spindle.0.at-speed" disabling the gui is not necessary.