Gordon completed his move to Sunderland for a fee which could rise to £9million on Wednesday.
The Black Cats manager told Sky Sports News: “We’ve worked very hard over the last five or six weeks since we started negotiations with Hearts.
“We got there in the end and we’re delighted. It’s a great signing for the club.
“When it’s hard I think it makes you more determined to get a player.
“We’ve had that from previous clubs when they’ve said players aren’t for sale, but I think everyone is for sale, everyone has a price.
“The best thing you can have at a club is competition for places and we’ve got that in most positions.
“We probably need one or two more players, but with the players I’ve got I’m delighted with the squad.”