Unfortunately I've also been caught out by this, where a GGL had already booked seats on a flight more than 10 months out where I also wanted to book redemptions. Not a major problem for me as it's on a route where there are many flights per day and I was able to book for the following flight.

The GGL welcome letter doesn't state this restriction, offering the following ".... you can take yourself... on precisely the reward flight you want, even if there are no reward flight seats available. Subject to availability."

I suspect the phrase "Subject to availability" covers the restriction which has caught you out. You can't get around it as far as I know so I recommend you just try to get the flight before or the flight after the one you want to take. It doesn't matter if there are other seats available in A or D or the other GGL "joker" classes.