Gwen couldn't protect him anymore.
The constant swarm of monsters, aliens and creatures unknown couldn't be stopped. They wouldn't politely separate home and Torchwood, her two different worlds, and see there was a line they shouldn't cross.
Gwen dreaded the day she came home to find the absence of Rhys's homely clatter of pots, pans, bottles, replaced by him seeping red onto the carpet they paid far too much for. Triumphant alien grinning over his lifeless body.
Her ring was returned in an unsealed envelope, propped against their wedding photo on the kitchen countertop. No letter or note accompanied it, although Gwen had spent the previous evening staring at a blank piece of paper, biro poised. But the words would not come. How could she explain that she now understood that Torchwood meant isolation, and isolation meant safety?
For the first time since joining the team she was acting like an adult.
Rhys life was worth more that her own selfish need for a family and love and release. That was the thought the drove her to the closet, packed her clothes neatly in a single holdall and closed the door quietly behind her on the way out.
All the monsters under her bed had turned out to be real, and only she, Jack, Ianto, Owen and Tosh stood in their way.