Gollings was first on the board in the second half converting his own try before replacement Nick Royle (Fylde) went over the line for his sixth try of the Series and sent England to a 45-0 victory.
In the second game, London Wasps winger Christian Wade scored two tries to help England beat Argentina 17-7.
Wade scored a double against Tunisia i the morning and repeated it in afternoon with a try in each half.
It was seven a piece at half time, the Army's Isoa Damudamu breaking the deadlock two minutes into the second half and Wade scoring his second try on the buzzer to give England the 17-7 victory.
Dan Norton then went over for two tries as Scotland were beaten 29-19 to set up a tough last-eight tie with New Zealand.
England Sevens Emirates Airline South Africa Sevens fixtures and results:
December 11, England 42-0 Tunisia
December 11, England 17 Argentina 7
December 11, Scotland 19 England 29