Muslims don't believe Torah to be corrupt nor do they deny the Torah and it is considered in the six articles of faith to believe in all revealed scriptures like Torah, Psalms etc . Also Muslim share a common patriarch Prophet Moses and believe in all of the subsequent prophets like Israel etc , yet why are Muslims considered to be gentiles?
In Judaism, someone is considered a gentile (or Jew) not based on his beliefs or actions. Being a Jew is having a status that attaches to a person under very specific circumstances; without those circumstances, he's a gentile. Specifically, to be a Jew one must be born to a Jewish mother or undergo the process of conversion that is specified in Jewish law. That is why most Muslims are considered gentiles.
I am not going to discuss whether it Islam thinks that the Torah text was corrupted (though from proffered quotes and discussions about which son of Abe was bound make me wonder) but I am going to answer the question exactly as asked "why are Muslims considered to be gentiles?" This is similar but not identical to "Why aren't Muslims Jewish"?
The answer is simple, really. Because they aren't Jewish. The word Jew (and therefore, its opposite word, the label for 'not-being') developed significantly after Moses' time -- it had to do with the divided kingdoms and the lineage of most of the people in the kingdom of Judah. When the term became the generalized title for "people of the Kingdom of Judah" it excluded people who were not of that kingdom. And when it became used (as it was in the scroll of Esther) to mean "those who subscribe to the code of religious law of the people of the Kingdom of Judah" it created the class of "Gentile" if not in name then in theory (the word gains its current meaning as a noun in the 14th century. So semantically, the deciding factor is about being a Jew (which can come about by proper conversion or through descent as codified by Jewish law).
So if one asks "is a Muslim a Jew, because semantically, many Muslims say that every Jew could be considered a Muslim in the broadest sense of the word as submitting to the will of god" the answer is "no." Being a member of the Jewish religion (and therefore not a gentile) has to do either with descent as defined by Jewish law or conversion as defined by Jewish law. It is not about a personal sense that one can find a link for himself to some historical personage.
Also note (as I have been musing about some of the comments on the Q and A's) -- Jews don't claim Moses as a biological patriarch, nor do we list him as a spiritual patriarch (the forefathers were Abraham, Isaac and Jacob) so finding a shared belief in him as a prophet or a great grandfather doesn't have an effect on one's status as Jewish.