The 19-year-old has been trialling with the club for the past few weeks, making appearances in all of the pre-season friendlies so far.
Those performances have earned him a one-year deal with the Stones.
Willard began his playing career with Arsenal between the ages of 9 and 15, before moving to the south coast where he was with The Saints for four years.
Saunders said: “I am delighted to get Harley on board for next season. He spent the early part of pre-season with the Gills and was recommended to us by Peter Taylor.
“He’s come in and done really well and his addition gives me options across the midfield. Despite his young age, he’s had an excellent grounding at Arsenal and Southampton and you can see that in his style of play”.