The Whites defeated Tottenham Hotspur 3-1 at Elland Road in the Premier League on Saturday lunchtime with the game enduring another controversial VAR moment.
Utility man Stuart Dallas opened the scoring for Marcelo Bielsa's men from close range before Son levelled for Spurs in a busy first half of action.
Kane then put the visitors in front with a clever finish over onrushing goalkeeper Illan Meslier only to see the offside flag deny celebrations.
Replays, though, showed the effort was a lot closer than initially thought with Kane appearing level to the naked eye on review.
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VAR intervened and remained with the on-field decision after the use of on-screen lines to determine that the England captain's boot was just beyond the last defender.
United went on to add goals through Patrick Bamford and substitute Rodrigo in what was an impressive performance in LS11 to earn all three points.
Spurs boss Ryan Mason hit out at the decision to rule Kane's goal out post-match and ex-Premier League match official Gallagher had some sympathy for his complaints.
"Week in and week out I'm asked this and I always default back to that the technology is there," Gallagher told Sky Sports' Ref Watch segment.
"The guys at Stockley Park process it. I'm not an expert in that technology but they were convinced that it was just offside and that's how the decision arose. They were convinced.
"It just shows how fine the margins are. When it was brought in VAR said offside or not offside - everyone thinks it's a great idea.
"Nearly two years down the line now people are going 'wow, I didn't realise that' but that's the technology we've got and that's the technology we've got to use at the moment."
Former Whites skipper Stephen Warnock added over the incident: "It's a frustration we've got to this point. It's arguably the closest call we've seen where the lines are on top of each other.
"It does beg the case of if it is that close then it is probably onside. It's very difficult to make that decision."