I've been trying to apply GGUs to a number of flights recently but had a few things I wanted to verify, if anyone can help:

1. I have a flight booked via British Airways Avios that says it is in T class. There's no way to apply a GGU to this, correct?
2. I have a roundtrip flight that was booked via American Airlines (cash) but is on Alaska metal and has my Alaska FF number attached. Is it possible to use a GGU on this or must the ticket have been purchased via Alaska?
3. I have GGU certificates that expire 12/31/23 but my flight is in January. The website won't let me apply them (says the code is invalid) and a phone agent says I must travel prior to the end of the year, not just apply them prior to the end of the year. Is that true?