All answered in the very comprehensive GUF thread
Q: I haven’t got a companion. Can I still use a GUF2?
Yes. You can use a GUF2 on your own for a single or return journey. The GUF2 cannot be divided meaning that you if you use it as a solo traveller it cannot be used again.
Q: Do I have to travel myself when I use the Gold Upgrade voucher like the Amex 2-4-1?
If you book with BA you can use the voucher for any other person or persons. However you must pay for the booking yourself either through your account on ba.com or through the Service Centre. If you book through a TA then the voucher owner must travel on the booking.