Why won’t Gitl marry Yitzchak?

One possible reason that Gitl doesn't want to marry Yitzchak could be that she's already had her fingers burned by her relationship to Avrom Morowitz. Avrom went to America three years ago, and Gitl hasn't heard from him since, even though he promised he would send for her. One gets the impression that this whole experience has put her off men for life, an impression confirmed later on by the fact that Gitl doesn't get married or have any children.
Speaking of which, Gitl's not particularly enamored about the fact that Yitzchak already has two children from a previous marriage. As it would appear that Gitl's someone who doesn't want children of her own, it's hardly surprising that she's unwilling to become a surrogate mother to Yitzchak's children. Their late mother has clearly left behind big shoes to fill, and Gitl's neither willing nor able to fill them.