Gladys Knight -- She's the Best Thing That Ever Happened to Old Carolina School

Gladys Knight is sinking a ton of her own cash into a $2 million project to save a North Carolina landmark -- a segregation era all Black school -- with ties to her family ... TMZ has learned.

Gladys tells TMZ she and her husband Billy McDowell are converting the abandoned building in Canton, NC ... where Billy's mother worked as a janitor. It's been sitting empty since the early 50s when schools were integrated there.

Gladys and Billy are renovating the old school into a shiny new public rec center -- and already spent $85,000 just to buy the building ... but hope this will help keep the history of the community alive.

They tell us they're actively raising funds to pay for the all upgrades ... which they estimate will cost a total of $2 mil.