The Gellert archive is housed at Indiana University, Bloomington, IN (also home to Bruce Conforth). The listing:

[United States, North and South Carolina, Georgia, Afro-Americans, 1920-1940] [sound recording]
  Gellert, Lawrence, 1898-1979.
  221 sound discs : analog, 78 rpm, mono. ; 7-10 in. + 4 sound tape reels (10 in.)
copies available at Blmgtn - Archives of Traditional Music (B-ATM)

In addition, the university library has copies of Gellert's books, his papers, copies of the issued sound recordings, and another apparently Gellert-related item whose relevance is uncertain:

[United States, 1966, 1968, 1969] [sound recording]
   6 sound tape reels : analog, 3 3/4 ips, 2 track, mono. ; 5 in. + documentation
copies available at Blmgtn - Archives of Traditional Music (B-ATM)

Bob