How to Control a PWM Gripper

I have my hobby robot gripper working and an Axis GUI slider to manually control/test it.  So now I need a hook to control the gripper from an eventual gcode program.

I'm thinking of having 3 states, Open, Close, Variable.  I'll need to output a float value, so is this best suited for G, M, O, or other code? 

Secondly are "aliases" possible in gcode?  For example "Gripper Open"  or "M65 Open" something relatively simple.

You could use the 'motion.analog-out' pins (M68):
linuxcnc.org/docs/html/gcode/m-code.html#mcode:m68

If you wanted you could define your own M-code or Subroutine that sets a particular analog output pin so you would not have to enter the E-word all the time.

We cannot pass strings in gcode and the only way to use something like that would be a subroutine (eg o<yoursubroutine>call)

I'm thinking about readability, so is this possible with subroutines...

Open CALL
Close CALL
Open 50 CALL (representing 50% full scale)

No that is not possible, subroutine calls need to use the 'O' Word (O <your_subroutine> CALL)
Note: lowercase lettering is also fine like in my last post. Subroutines can also accept parameter values (O <your_subroutine> CALL [123] [456] ...)
Remapped Mcodes can also pass P and Q words: (M456 P1 Q2)