Hello @David-L - good call out on our documentation. I need to go get it updated to include this.
After a bit of digging there were technical obstacles to including a voice vlan config on the GX, so this is a GS only feature. I'll make sure that our documentation also reflects that as well, to remove the confusion.
Hardware -> Switch -> Switch Ports -> Choose a port -> Settings -> Adv. Settings
Finally, to somewhat offset the loss of the voice vlan config for the GX, we do support DHCP options on the GX product now (so those already using voice vlans downstream can start using DHCP options).
You can use the preset options we have built in, or use custom ones:
Networks -> [Wired network] -> Settings -> Enable/Disable DHCP server