The club also revealed that defenceman William Lagesson has been assigned to the Bakersfield Condors of the AHL.
Gagner returned to the Oilers in a trade with the Vancouver Canucks for Ryan Spooner last February.
The 30-year-old — who was drafted by Edmonton sixth overall in the 2007 NHL Draft and spent the first seven seasons of his career with the club — recorded 13 points across 32 games with the Canucks and Oilers combined in 2018-19.
He has collected 446 points during 12 years in the league with stops also in Arizona, Philadelphia and Columbus.
Manning, 29, had four points in 39 games with both the Oilers and Chicago Blackhawks last season.