I made GGL a few months ago and have been trying to work out what the real benefits are. The joker is great, and probably the best benefit. I have also managed to get upgraded from J to F in 2 of my last 3 flights, which is certainly better than the 0 upgrades i got in the last 4 years as a "normal" gold member. The extra guest into the lounges is also a nice perk.

However, its not the great leap that I had anticipated. I know GGL is really only a sub-tier but its rather sad that the majority of BA staff that I speak to have no knowledge of GGL at all. I even had a purser asking me what GGL next to my name meant!!

Whether its worth it for you compared to retaining gold on your other FFP, I guess that's down to how many flights you are going to take, and with who.
If you will fly enough with BA, then I would recommend going for it.