If a Jewish man were to father a child with a gentile would they be allowed to consider an abortion or would that be disallowed in the Halachic literature ?
I see no reason why it should be any different than the general discussion about abortion.
Until an embryo is implanted: of no halachic significance. You couldn't break shabbos to save a frozen embryo, for instance.
From implantation till 40 days: Rabbi Moshe Feinstein vociferously opposed in virtually all circumstances, unless this baby is seriously threatening the life of the mother. Other poskim have some room for leniencies. I've also heard from Rabbi Weiner that at this stage it may be stricter for Jews than for non-Jews (yes you read that correctly).
40 days till 6 months: Allowed if threatening the mother; Rabbi Joseph Dov Soloveichik allowed in the case of Tay Sachs G-d forbid.
6 months till the head emerges: Forbidden by everyone, unless threatening the life of the mother.
Head emerges: even if threatening the life of the mother, "we don't push one life in favor of another", let nature take its course.