Just to be clear: you are correct in that the 'none' option is not available for OSC.
We have a ticket going to have that fixed by our engineers.

Unfortunately I don't have an ETA on that for you.
Now, I have done a little more research, and from what I can tell, the GrandMA3 wants this formatted as:
/eos/sequence/1/cue/3/go+ with the argument
1 (which enables the command)
Therefore, in theory, you should be able to send this as an 'integer' and use '1' as the set value.
A second could be to send this as a formatted UDP message, as a string. That will take some trial and tweaking.
And a third option, if applicable, could be to use another VP device in your system, such as a CueCore to 'relay' the message for you.
Sorry for the inconvenience!