With the score at 0-0, Gabbiadini tapped in a cross from Cedric Soares from close range, but was denied by the offside flag, even though only team-mate Ryan Bertrand - who did not make a move towards the ball – had strayed beyond the last defender.
United went on to take a 2-0 lead through Zlatan Ibrahimovic and Jesse Lingard before Gabbiadini's brace levelled matters. However, Ibrahimovic's late header settled a dramatic contest to give United a 3-2 win.
Speaking to talkSPORT about the decision, Le Tissier said: "The history of football shows you the bigger clubs get a lot more decisions in their favour than the small ones.
"[If United had scored] it wouldn't have been given offside, I genuinely think that. The flag wouldn't have gone up in the first place.
"There are certain decisions where the margins are so close, you can understand why mistakes are made.
"I still can't understand how he can give Gabbiadini offside when the ball was played by Cedric, he couldn't even see Gabbiadini because the defender was in the way.
"Surely that gives him a clue that, actually, Gabbiadini wasn't offside."
Le Tissier was similarly emphatic in his criticism of his decision when covering the game for Sky Sports, saying: "This is a cup final, he has to get these decisions right and that decision was disgusting."