'Orange County Housewife' Gretchen Rossi -- Tamra Barney's Engagement Is a SHAM

"Real Housewives of Orange County" star Gretchen Rossi is telling friends ... Tamra Barney's new engagement is based on a money-grubbing lie -- manufactured by show producers desperate for a dramatic finale.

Sources close to Gretchen tell TMZ, producers originally approached Gretchen's BF Slade about proposing -- hoping he'd pop the question in time for the show's big finale this year. We're told producers even attempted to bribe Slade with an exotic proposal destination ... and a FREE RING.

We're told Slade turned the offer down -- convinced the public already hated him enough without a fake proposal -- so producers turned to Tamra and her BF Eddie Judge out of spite.

We're told Tamra and Eddie instantly snatched up the proposal package -- and her whole proposal was then conveniently filmed for the show ... at the same exotic location that was allegedly offered to Slade.