Stanaway arrives to replace him full-time, along with new major sponsor Boost Mobile following the exit of Wilson Security.
Tander made his debut with Garry Rogers Motorsport in 1998, winning Bathurst two years later before joining the Walkinshaw empire in 2005.
He returned to GRM in 2017 on a two-year deal that was extended into '19, which would have been his 21st season in Supercars.
Following Stanaway's split with Tickford Racing after a torrid rookie campaign, it is understood Boost took an active interest in finding a Supercars lifeline for the Kiwi.
Stanaway will partner James Golding, the pair having finished 25th and 22nd as rookies in 2018.
"It obviously comes as a surprise to most that Garth will not be driving at GRM in 2019," Rogers said.
"Garth has rewarded the GRM team some of the biggest moments in Motorsport including the 2000 Bathurst 1000 win alongside Jason Bargwanna, second in the championship behind Mark Skaife in the same year and the success of the Monaro 427 in the Bathurst 24 Hour races of 2002 and more.
"On Garth’s return to GRM in 2017 he was guaranteed 2 years of full-time driving and a management role following that.
"There were many aspects to my decision, including that Garth was nearing the end of his career and has expressed that 2019 could be his final full-time season as a driver.
"I was very conscious of the fact that I would be both the one that started and ended Garth’s full-time Supercar career and this weighed heavily on me.
"In a perfect world I would absolutely have loved to give Garth the send off that such a champion deserves, but I never lost sight that motorsport, although centred around the driver, is about the team and not any one individual."
Tander was to become the oldest full-time driver in Supercars this year following Craig Lowndes' retirement, a mantle that now goes to James Courtney.