There is a guide here too

BA Gold Upgrade Voucher Discussion and Wiki Thread

Q: Can I use the voucher on an itinerary that includes non-BA flights?
With a booking direct with BA this is fine. For a TA booking you can’t include non-BA flights. However, there are the occasional reports of a few slipping through where some Travel Agents have managed to apply a GUF to a booking which includes both BA and non-BA flights on BA codeshares.