Were non-Jews condemned to hell in the Old Testament?

Yes, but not because they were Gentiles.  Both Jew and Gentile who find themselves condemned to eternal hell in both Old and New Testament will suffer that fate because of unrepentant sin (Rom. 2:3-11).  God does not wish this for anyone because he loves all (John 3:16).  That is why, for example, he sent the prophet Jonah to preach to non-Jews: in the hopes that they would repent and find forgiveness from him (Jonah 3:1-10; 4:1-2, 11).