My Moto G4 Plus got the Nougat update around 29th or 30th Dec 2016 (India). So I suppose the G5 Plus should be getting the Oreo around that time along with the G4 Plus.
The G4 Plus is getting security patches every 3 months and is now on 1st June security patch.But my G5 Plus is still on the March security patch which it got out of the box when I received it on the 11th of August.