Speed, 42, finished his playing career in South Yorkshire with Sheffield United following a record-breaking career in the Premier League with Leeds, Newcastle, Everton and Bolton where he made well over 500 appearances.
After a brief spell as Blades caretaker boss, Speed left to manage the Welsh National team last year where he'd just achieved three successive victories.
Manager Andy Scott: "He was a fantastic bloke, and will be a great loss to the game. He was the model professional, and lived his life as a real pro. Everyone admired Gary, it's a big shock and we'd like to offer our condolences to his family. In the summer we were in Portugal and Gary was there with Alan Shearer. We didn't want to bother him, but he came over to me and we had a chat. He went out of his way to ask how things were going, and that's just the sort of bloke he was."
Coach Andy Dibble: "I've known Gary for over 20 years, from playing against him, to being involved with the Welsh team together and I can say he's one of football's true gentlemen. My heart goes out to his family and his friends, he will be sadly missed."
Skipper Ryan Cresswell: "When I first started getting in and around the first team at Sheffield United, Gary had just joined the club. Although there wasn't much interaction on the pitch between us, he was great off it. He was such a good guy, and a professional that professionals looked up to. My thoughts are with the family, it's just devastating news."
*Words from www.themillers.co.uk