That leaves the 6ft 1ins, 15st 12lbs Glynneath youngster competing with Mike Phillips and Richie Rees for the Wales jersey, Dragons scrum-half Wayne Evans not included in Gatland's selectorial thinking.
"We have been impressed with him," said Gatland of Knoyle.
"As a youngster he has come on really well and I know he has been spoken of in recent seasons as someone to watch.
"I'm a great believer in 'if you are good enough you are old enough' and that applies to the likes of both Tom Prydie and Tavis Knoyle.
"We have gone very much for continuity of selection and kept the squad small, but there are also a couple of youngsters in there who we are going to stick by."
Archive, May 2009: 'Awesome' year for young scrum-half
Knoyle played his youth rugby for Glynneath and has impressed for Wales at Under-20, U18 and U16 levels.
Having graduated to the Neath side he was wanted by the Ospreys, with rumoured interest also coming from Leicester and Harlequins.
But he chose to join the Scarlets on a two-year deal last summer, coach Nigel Davies saying at the time that he was "as excited about Tavis as I was about Dwayne Peel and Mike Phillips at 18".
Davies has six of his squad in the Wales party, uncapped trio Knoyle, Rob McCusker and Ken Owens joining Stephen Jones, Matthew Rees and Jonathan Davies.
Our policy of exposing these younger Scarlets players to top-flight rugby and playing them in the big pressure games is paying off and has allowed them to mature
Scarlets coach Nigel Davies
"This is great news for these boys, particularly Tavis, Ken and Rob who have really shown their talent, commitment and determination to improve this season with us," said Davies.
"It will be an invaluable experience for them to tour New Zealand and spend time with the Wales set-up and all the senior players.
"It will only enhance what they can bring back to the region next season.
"Our policy of exposing these younger Scarlets players to top-flight rugby and playing them in the big pressure games is paying off and has allowed them to mature, gain some great experience and prove themselves. We wish them all the best for these tests."
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