Elizabeth asks him why he's leaving. He tells her because the burden of the bad as well as the good that he's done adds up, with the bad becoming a little too much to bear. We probably know what tipped the balance - lying to Philip about his son.
And when Philip comes to see him, at Gabriel's request, Gabriel lies to Philip again - a lie of omission, in still not telling about his son. But he does impart a surprising nugget of truth to Philip, in his quiet, heartfelt words to Philip that Philip was right in not wanting Paige to go into the business. Too much possible bad to outweigh the good.
And Paige had a powerful night herself, in effect moving ever closer to her parents' line of work, by breaking up with Matthew. She doesn't want to keep lying to him. But, if she has a low tolerance for lying, then maybe she's not cut out to be a spy after all.
Frank Langella has been a world-class actor all of his life, from his brilliant portrayal of Dracula on Broadway (which my wife and I saw in 1977) to every film he's ever been in. He brought all of that to Gabriel in The Americans. His conversation with Paige at the beginning of the hour was one of the best in the entire series.
Like Elizabeth and Philip, I don't want to see him go.
And see also The Americans: True and Deep ... The Americans 1.4: Preventing World War III ... The Americans 1.11: Elizabeth's Evolution ... The Americans Season 1 Finale: Excellent with One Exception