Here is the full history of recordings of Kumbaya in the AFC archive, and other early evidence.

Robert W. Gordon did indeed record something that looks like "Kumbaya," in Georgia in 1926. It was the sixth verse of a longer spiritual, but he was convinced enough it was related to other recordings he made of songs called "Come by Here" that he cross-referenced it to those songs. Unfortunately, these other "Come By Here" cylinders were damaged, and so are unplayable. However, they were all recorded in Georgia and South Carolina, so a Gullah provenance for at least some versions of the song is quite plausible.

The 1926 recording must be the one Pete Seeger was remembering. It has been transcribed by AFC reference librarian Todd Harvey, and is available to researchers in the AFC reading room.

Cards for the other early playable recordings in the archive can be found using the link I gave above. They date from 1936 on.

In 1931, the Society for the Preservation of Spirituals published the book The Carolina Low-Country, which contains a version called "Come by Yuh." The exact date and location are not mentioned, but given the activities of the group's members, it must have been noted between 1922 and 1931.

As an aside, the Lum Chee-Hoo article in Kodaly Envoy, referenced in Richie's post above, gets it mostly right. The newspaper accounts of Lum's work conflated Gordon's 1926 recording with Lomax's disc recording a decade later. In any case, both existed before Frey wrote or published his song "Come By Here."