The 24-year-old has been on loan at the Keepmoat Stadium since October, after hardly figuring under Alan Pardew
At the time of his move, Saints agreed the Championship side could sign him on a free transfer when his temporary deal expired.
Doncaster have been attempting to agree on personal terms with the player over the last few days.
That has taken longer than Rovers manager Sean O'Driscoll would have liked.
As a result, Gillett has now returned to St Mary's, with his loan having expired on Tuesday.
But Doncaster remain hopeful he will soon become a full-time fixture at the Keepmoat.
Former Bournemouth boss O'Driscoll said: "We've offered him a contract and everything is agreed with Southampton so we are just waiting to hear back from Simon.