However, Gabbiadini, who had only been on the pitch for nine minutes, struck from fellow substitute Joshua Sims' 90th-minute cross to deny the Clarets a first win in 11 league games.
"I think in the way that we equalised we can be happy because it was really difficult, after conceding against this side who don't concede too much themselves," said Saints boss Mauricio Pellegrino.
"But, obviously, we have to analyse well, because we have to improve in different aspects – how can we play against those teams who will press and are really aggressive physically, and who try to put you under pressure all the time?
"I think our concentration was good, and our intensity too, but I think we were a little bit nervous in some moments. When you don't want to take any risks sometimes the ball is always in the air and for us it was difficult to play.
"We had a couple of chances to equalise and at the end we got this goal that allowed us to get one point. It's not the best, but in this moment one point can be the big difference at the end."