Gatland is approaching the game with an experimental team in mind with Williams himself switching from wing to full-back.
Flanker Dan Lydiate will captain the side which also includes a debut run out at number 8 for Cardiff's second row Seb Davies.
Four players will make their first starts for Wales in Saturday's fixture, with Olympic medallist and flanker Sam Cross, prop Leon Brown, second-row Adam Beard and centre Owen Watkin all getting the nod from Gatland.
Dragons' Elliott Dee is the only uncapped player in the 23 man squad.
Gatland says it is important to rotate his squad in these autumn internationals.
"We felt this week was really important to give a number of the squad as much exposure as possible to Test match rugby," said Gatland.
"It is an exciting opportunity for some of the younger guys to take the field and show us what they are capable of."