Cardiff Blues scrum-half Williams, 23, will make his debut against South Africa in Washington DC on Saturday.
Gatland feels Williams still has plenty of room for improvement but he says his form at club level means he fully deserves a chance to impress on the international stage.
"He's forced his way in," said Gatland. "He's done his time with Cardiff Blues.
"He's very combative and competitive and he's got a little bit of an edge about him as a player. I'm looking forward to seeing him at this next level. There's still a lot of development for him to do over the next 18 months or so but he's one for the future."
Gatland has made 13 changes to the side from the Six Nations win over France in Cardiff in March for the Test against the Springboks, although Williams is the only uncapped player in the team.
The New Zealand coach has urged his players to make the most of their chance.
"Players aren't just here to make up the numbers or hold tackle bags, we want everyone to get a chance,' he said.
"So if you look at the squad, it's mainly all those players who were with us for the first week, obviously the Scarlets were preparing for the final.
"Those players have had a couple of weeks training together in preparation so most of them are ready from the time that we've had together."