An offensive term for a Gentile is Shiksa if used for a woman and Shegetz if used for a man (literally meaning abomination). Shegetz is usually used to describe an anti-Semitic man. Shiksa is usually used to describe a Goy woman who is married to or dating a Jewish man.
It should be noted that while all of these terms may not be meant offensively by their users, they will almost always be perceived as offensive by those they are used in reference to.