Wales kick off their campaign against Ireland on February 3 and Gardiner is among 36 players named for training ahead of the Championship. A 30-man squad will be finalised at the end of next week.
Wales Under 20s Extended Training Squad
Forwards: Rob Evans (Scarlets), Tom Davies (Blues), Samson Lee (Scarlets), Dan Suter (Ospreys), WillGriff John (Blues), Kirby Myhill (Scarlets), Jack Dando (Blues ), Darran Harris (Blues), Jac Hopkins (Cardiff), Jamie Sollis (Dragons), Matthew Screech (Blues), Jack Jones (Ospreys), Rhodri Hughes (Ospreys), Jonathan Gardiner (Leicester Tigers), Ellis Jenkins (Blues), Daniel Thomas (Scarlets), Dan Crimmins (Cardiff), Ieuan Jones (Dragons), Rueben Tucker (Blues), Dan Baker (Ospreys).
Backs: Jonathan Evans (Dragons), Joshua Davies (Dragons), Jack Moates (London Irish), Jordan Williams (Scarlets), Sam Davies (Ospreys), Lewis Williams (Blues), Rheon James (Scarlets), Thomas Pascoe (Blues), Cory Allen (Blues), Nathan Edwards (Bridgend), Luke Williams (Dragons), Luke Morgan (Ospreys), Iolo Evans (Scarlets), Jake Randall (Scarlets), Tom Prydie (Ospreys), Ross Jones (Ospreys).