The 22-year-old grabbed the only goal of the game late in the first half as he cut in from the right to fire a deflected shot past Nikolay Mihaylov in the Bulgaria goal and end the Euro 2012 qualifying campaign on a high. The recently-crowned Wales Footballer of the Year also hit the bar with a fierce volley.
Speed said: "Gareth was phenomenal again. The pitch out there tonight did not suit his game, I have played on pitches like that and it's heavy and dead, the grass was long and we said to the players at training last night that if you run with the ball on that you are going to get caught, so to put in a performance like that is phenomenal."
The Wales boss added: "He is a match winner, but that said you have to give your match winners a platform to win games for you, and everyone else gave him a platform and he puts the hard work in too. That's how we will achieve things and he works harder than anyone and that's the situation we need if we are to keep improving."
The win gave Wales back-to-back victories in competitive fixtures for the first time since 2005, and the manner in which they ground out the result was particularly impressive, with Bale, captain Aaron Ramsey and Joe Allen once again outstanding in midfield.
The poor pitch and a sparse crowd of just over a thousand, due to a boycott by home fans disgruntled with their sides current winless run of seven games, provided difficult hurdles for Speed's young squad to overcome.
And the manager admitted that his players had surpassed his own expectations with victory, and praised their attitude in a testing environment.
He said: "I am really delighted. I was apprehensive before the game to be honest. We have come so far so soon that before the game I was thinking this might be one game too far because of the circumstances the players found themselves in.
"Many of them have not experienced it before. To come to Bulgaria, a place that is difficult to come to and try to grind out a result.
"I would have settled for a draw before the game. I wanted a positive result of any kind because we need this experience to make us better in the future. So to get a win; I did not expect it but it was brilliant."