The Blues assistant boss admitted it came at a time when he needed to get a message to his players.
Play was momentarily stopped when the drone was spotted by the referee hovering above Fratton Park.
The players were ordered to the side of the pitch.
As a result of the delay Gallen was able to speak to them.
He said: ‘I could see it but I just didn’t take any notice of it!
‘I used the break to go onto the pitch and speak to a few players.
‘I was pleased to see it because we were trying to get our markers sorted out from set-pieces as they were bringing on George Miller, another centre-forward.
‘We needed to work out who was going to pick people up because they were going to have too many so I used that moment to go onto the pitch and talk to the players.’
It was first for Gallen who didn’t realise the match would have to be stopped for a drone flying.
He added: ‘I haven't experienced anything like that.
‘Apparently I’m a bit naive because I didn’t know if there’s a drone we’d have to stop.
‘I just thought we’d carry on.
‘I’ve never been involved in that sort of situation before.’