Here are the listings from the great San Francisco Express Times, vol. 2 no. 32 (August 21, 1969), p. unk. There's lots of interest here, of course. But I have circled the item that interests me most greatly. It's under the listings for Sunday, August 24, 1969, and reads as follows:
Hippy Hill: Trans-Cultural Rip-Offs, Inc. presents Steve Gaskin & the Grateful Dead in concert with Shiva Fellowship. Bring dope (the sacrament) and good vibes. noon. free.
"Hippy Hill", a.k.a. Hippie Hill, is apparently at the far eastern edge of Golden Gate Park, close to the entry from the Haight-Ashbury neighborhood. It seems like a perfectly good place to go share a sacrament and a free show by the Dead.
The Grateful Dead List traditionally shows the GD at Paradise Valley Resort, Squamish, BC, at the Vancouver Pop Festival [dead.net | deadlists]. There is a handbill, but I have never seen any eyewitness or other accounts of the GD's performance. This listing makes much sense given the band's presence in the Northwest (Bullfrog Festival in Oregon on 8/23/69).
That said, I find the listing in the Express Times to have a lot of verisimilitude. I have been reading a lot of stuff from ca. August 1969 around The Common and the Wild West Festival. The Dead doing a free gig in the Park makes sense in the context of all the shit that was going down, about which I will be writing, soon.