JGB fans certainly know this one:
On official releases, it's usually credited to Lightnin' Hopkins, but this is clearly the version they're covering. I can't remember which tape it is, but there's at least one recording where you can clearly hear someone (Kahn, I think) refer to this as a B.B. King tune.
And, while B.B. King didn't write or originally perform "The Thrill is Gone" either, it's another song that everyone associates with him. Garcia sang this one as well. I always think of Garcia/Grisman's work as casual get-togethers at Grisman's home studio, but I just remembered that actually made a real music video for this one.