The 192cm defender, who was delisted by the Cats last month after just 13 senior games, has been training with the Demons in the hope of being thrown a lifeline.
A Melbourne spokesperson confirmed that Gillies was training with the club.
The Demons could view Gillies as a replacement for veteran defender Jared Rivers, who joined Geelong as a free agent.
The Cats announced on Monday that Rivers has been handed No. 25 guernsey, which was previously worn by Gillies.
If Gillies impresses Melbourne, he can be snapped up as a delisted free agent. If not, the former Cat is likely to head to the NAB AFL Draft.
"We're going to let the dust settle from the trade period ... then we'll see what comes over the next couple of weeks in regards to hopefully getting picked up in the national draft," Gillies' manager Tim Hazell told AFL.com.au last month.
Gillies, 22, was often touted as a ready-made replacement at Geelong for champion backman Matthew Scarlett, who retired this year.
He was selected by the Cats with pick 33 in the 2008 NAB AFL Draft.
Click here to view Tom Gillies' career stats
A year ago Gillies was targeted by Hawthorn, but Geelong was determined to retain him, so he stayed at Simonds Stadium for another season.
He played just five AFL games this year but showed glimpses of his ability, especially in his 19-possession, eight mark effort against Melbourne in round eight, and his 18-touch and seven mark game against Greater Western Sydney in round 13.
He spent the remainder of the season in the reserves and was a part of the Geelong team that beat Port Melbourne in the VFL Grand Final.
Melbourne could be out to boost its defensive depth after the departure of Rivers. Last week, former Brisbane Lion Cheynee Stiller was given permission to train with the Demons.
But the former half-back had a change of heart, telling the Demons that he no longer had the passion required to move to Melbourne in an effort to continue at AFL level.
Niall Seewang is a sub-editor at AFL Media. Follow him on Twitter @AFLN_Seewang
