At first, we only know Ellen James as the inspiration behind the Ellen Jamesians. And guess what? Garp hates the Ellen Jamesians. He has a strong distaste for political radicalism in any form, even if he sympathizes with their cause, as he does with the Ellen Jamesians. It turns out that Ellen James herself hates the cult-like group, too—she wishes "she could talk" (17.238) and resents them for muting themselves.
But Ellen, too, is seeing with a close-minded viewpoint. Although the actions of the Ellen Jamesians are by no means sane, neither are the hardships they have to endure. Ellen realizes that she's lost sight of the true enemy—those who would use their power over others for evil—and instead resented people who are generally pretty well-intentioned.
This revelation is a turning point for Ellen, and it's also a signal that she is mature beyond her years (being raped and having your tongue cut out, then turned into a beacon of feminist discontent will do that to a girl). As Garp well knows, that's a great asset for a burgeoning writer, which is great, since this is exactly what Ellen wants to do with her life.