Rugby Union: Williams won over by coach's persuasive skills.
And Williams, who had been adamant he would not make a Test comeback, conceded that it had not been an easy decision.
"I was convinced it was the right decision and there was no turning back," he said.
"But the turning point - in terms of realising I'd made a mistake - was when I played for the Baa-Baas against South Africa at Twickenham.
"Coming away from there I realised just how much I loved being part of that international environment.
"I also realised that I still had something to offer and that the desire was still in me."
And Williams revealed the persuasive powers of Gatland were also key.
"I thought he wasn't going to ring, so it was pointless speculating about it," added Williams.
"But then he rang and we arranged to meet up.
"When I sat down with him, he sold it so well. I had a long chat with him and the way he spoke about things was just so persuasive.
"It really brought to the surface what I had been thinking and the doubts I had been having.
"I knew then that I was going to change my mind. I rang Warren on Sunday and said I was available."