Fleeton's departure comes after two games at AFL level for the Giants, having made his debut in Round 9 this year against Collingwood at the MCG.
The 21-year-old would play the following week in a loss against St Kilda before returning to the VFL, where he would piece together a strong season to earn VFL Team of the Year honours.
Gillbee would not feature at senior level for the Giants in his 12 months with the club, playing all 14 of his games in orange and charcoal in the VFL where he would average 15.9 disposals and 4.6 marks.
Just days on from the Giants' preliminary final exit in a loss to the Magpies, GWS announced the two departures on Monday afternoon.
“It's always a tough time of year to let players go that have contributed to the club,” Giants football boss Jason McCartney said.
“Cam has spent three seasons with us and has been a steady and reliable member of our VFL side but was just unable to break into the AFL side consistently.
“Jason has been with us for a season after being selected as a category B rookie and was unfortunately unable to make his mark.
“Both Cam and Jason have been great additions off the field who have contributed to our club, and we wish them both well in their next ventures.”