Judging on the timeline of GK&TPs and Motown, according to Don't Forget the Motor City, "Neither One of Us" was released on December 26, 1972 for promo.

Then in January 1973, GK&TPs tried to negotiate a better contract with Motown Records as their contract had expired that month [[I guess they signed to Motown in January 1966?) and negotiations broke down after several days due to them not seeing eye to eye on things [[how prophetic that the lyrics to Neither One of Us had the words "there can be no way this can have a happy ending").

The group is allowed to leave the label and in February 1973, it's reported on Billboard that the group has signed with Buddah Records. "Neither One of Us" topped the charts in March of 1973.

So "Neither One of Us" definitely was the last song GK&TPs released while still on Motown while they released at least three other singles posthumously.

All I can think of is, "wow, what a way to go out". There have been many legendary Motown acts who leave the label, most without much fanfare but I never read about such a dramatic exit as Gladys Knight & the Pips'.