I've figured out the issue:
Our share is located on a NetApp SAN. With ONTAP v9.7P8 and onwards, there is a bug that closes CIFS sessions due to idle timeout for Linux clients. The default timeout is 900 seconds. I changed the idle timeout to 2 days and our Gateway backups work after that.
NetApp command: vserver cifs options modify -vserver XXXX -client-session-timeout
Hope this helps anyone else that may encounter this issue.