Updated: the SFC Datebook of April 5, 1968 (p. 47) shows John Lee Hooker, Loading Zone, Mother Earth. Given the two conflicting pieces of print, I'll prefer the later-published one (SFC) to the earlier-published one (Express Times issue of ca. 4/3/68) (lex posterior derogat priori).
Corry's tour itinerary shows an open Friday night for GD/Garcia on April 5, 1968.
Randomly, I just found a listing for the GD and Chuck Berry at the Carousel that night. Here it is.
This is one of the interesting thing about cross-validating from the Barb to the Express Times and other sources - sometimes interesting things pop up!
The Chicken lists John Lee Hooker, Loading Zone and Mother Earth.
update: The Chicken was right all along, and I think there was no hidden GD show this night, at least not at Carousel.
Interesting that this was ever on any calendar at all, though. I wonder what the GD ended up doing this night instead of gigging, as at-one-time advertised?