Early on in her childhood she got a job as a chambermaid to House Umbridge. A childhood that would get cut short rather abruptly by the fascinations of Arthur Umbridge. He forced a child upon her when she was fourteen.
Arthur died the year following the birth of James, his son with Gwynneth. But the scandal was quickly discovered and the Umbridges took Gwynneth under their wing. Her bastard son was accepted into the family and even named heir in the wake of his father's death.
Gwynneth became a lady-in-waiting for Lady Margeret. As of the elderly woman's death in 1014, Gwynneth has taken over some of her duties although she has never been given a title.
She has never married and is not known to have lovers. Her relationship with her son seems sound, but she is such a meek, quiet little thing that hardly anyone even notices her.
Gwynneth is well-aware that if Harold Umbridge remarries and has a child, James will no longer be heir. After Lady Margeret's death, she even offered herself to Lord Umbridge. He made it abundantly clear if he were to marry again, it would not be to some lowborn peasant. She hasn't broached the topic again.
There was news of her being pregnant a few months ago, with an unknown father, breaking the rumor that she does not take lovers. However the pregnancy never came to full term.