Controlling Ion Cue List with Gio @5?

It's easy using OSC strings. Configure an OSC connection between the 2 systems and use Cue x {Execute} {String} to send whatever commands you like. I'd probably use a linked secondary cue list to help keep things organized on the Gio but that's a personal preference.

i think you should be able to use the setting Setup > System > Show Control > OSC of the OSC Cue Recv String on your ion (assuming the operator sits at the @5). the assumes that your two cuelists are numbered equally. you don't need to have all the cue on both consoles. but the ion will only fire cues if the @5 has a cue with the same number.

if you go to the setup place i mentioned above and hover your mouse pointer over that field you will get a small tool tip that shows you how it works