National coach Steve Hansen named Willaims as the 32nd member of his squad over Pontypridd youngster Matthew Rees, Bath veteran Jonathan Humphreys and Neath prop Duncan Jones.
Williams is the the third uncapped player in the squad, joining Llanelli centre Matthew Watkins and Pontypridd fly-half Ceri Sweeney.
Wales boss Steve Hansen said of the young forward: “Gareth is very good at the line-out, and is very mobile.”
And 24-year-old Williams is now set to be on the bench as back-up for Mefin Davies at Stadio Flaminio.
