Severance

I wonder if she lied about Gemma being killed to get back at Lumon

Cobel is working for Lumon in the last episode. To what end? Who knows? How else did she know about Mark S an Helly R's romance? She was gone from the Severed Floor long before that was a thing. The birthing shed was the very first interaction she'd had with Mark S since her firing because of the OTC event.

The only way she would have known is if someone from Lumon told her. The only reason Lumon would have told her is because she was brought back in the fold.