Anyone expecting a repeat of the thrilling third round tie when City came from behind to dump out Spurs would have been disappointed with a game which offered few talking points until right at the death when Gerrbrand failed to control a high bouncing goal to let in Kenwyne Jones.
The young striker left unmarked as he rifled a close-range shot beyond the unprotected Rab Douglas.
Jones, who is World Cup-bound with Trinidad & Tobago, had been on the pitch only a short while having come on as an 84th minute substitute for Dexter Blackstock.
In the early stages City defender Richard Stearman was fortunate to escape with only a yellow card after he scythed down Marian Pahars and the Latvian international almost exacted sweet revenge when he headed the resulting free-kick fractionally wide.
Leicester first threatened when skipper Patrick McCarthy headed over from a Ryan Smith corner and McCarthy went close again on 17 minutes with a looping header after Gerrbrand had played a half-cleared ball back into the penalty area.
Although the pace of the game continued to be fast and furious, neither side was able to create any more clear chances before half-time.
Saints fashioned the first chance after the restart when Pahars should have done better than blaze his far post effort high and wide.
Then on 57 minutes came what proved to be City's best chance when Stephen Hughes' clever ball split the visitors defence to leave Elvis Hammond one-on-one with the keeper.
The City striker looked favourite to score but Bartosz Bialkowski remained calm by standing his ground as he kept out Hammond's shot with an outstretched hand.
Alan Maybury then elected to go for glory when he blasted a shot wide when a better option would have been to pass to Iain Hume who was standing unmarked in front of goal.
Southampton almost broke the deadlock on 73 minutes when Nathan Dyer raced clear to square the ball for Blackstock who bundled his near post effort just wide.
After that the game appeared destined for a replay at St Mary's Stadium – until Gerrbrand's howler let in Jones for his late, late winner.