Yes, there was indeed a gasket for it.
It was assigned these part numbers:
2508 (1948-1961) for oldest Suds-O-Gun
250961 (1961-1983) for newer Suds-O-Gun
250783 (1984-1990) improved part for 1961-style Suds-O-Gun (introduced during the Heritage 84 run)
After it was redesigned for the G3 run, there was no gasket.
~Ben