Say you have a Christian* or a Muslim (or a member of any other monotheistic religion) who accepts the Seven Noachide Laws, and (as per Mishneh Torah, Hil. Melachim 8:11) does so because he believes that G-d (re)gave them through Moshe.
However, he also continues to believe ideas from his own religion that are against the Torah or even inimical to it, such as that most of the mitzvos are no longer applicable (for anyone, Jews or non-Jews) after the crucifixion, or that the Torah as we have it is corrupt and that the Koran is the true revelation of G-d's word, etc.
Would such a person still be considered a righteous gentile? True that Rambam says (ibid. 10:9) that "we are not to allow [gerei toshav] to innovate a religion and come up with their own mitzvos" - but perhaps that refers specifically to rituals and ceremonies, not ideology. On the other hand, one could argue that such beliefs make the acceptance of the Seven Laws incomplete.