Gallagher was named skipper against Bournemouth in a 0-0 draw at the Vitality Stadium last Sunday.
But the Chelsea star was shocked to learn that he had been given the armband before kick-off.
Gallagher said: "Against Bournemouth I had no idea I was captain. I didn't think I would be, if I'm being honest.
"But I lifted up my kit to arrange it before I put it on and the armband was there. I said to the kitman 'is this mine' and he said 'yeah', so that's how I found out.
"To captain the side on a couple of occasions is really special. My parents are really proud too.
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"There was no phone service in the changing room, so they didn't get my message. They just saw me on the pitch with the armband, but I think that was a nicer surprise anyway."
Reece James has taken on the role as captain since Cesar Azpilicueta's departure but he is out injured while vice-captain Ben Chilwell was on the bench.
And Pochettino has told Gallagher that his Chelsea roots have helped him earn the armband.
The Argentine coach said in his press conference on Friday: "He was captain against Wimbledon also.
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"He is supposed to be captain. It's nice.
"It is normal if you see the starting XI versus Bournemouth, he was also captain versus Wimbledon, it's not a big thing to create a party.
"He can be a captain, he is from Chelsea and it is perfect for him to represent the team."
It comes after a summer where the England international was heavily linked with a move away with Tottenham and West Ham both interested.
But Pochettino remains a big fan of the midfielder and he has started every match this season.