When did Gentiles become “unclean”? 

When the Israelites first entered the Promised Land (in the time of Moses) they were supposed to empty it of all Gentiles. However, they didn’t and it resulted in the Jews adopting the religious views of their neighbors which led to their destruction. 

In later years, the Jews became fanatical about separating themselves from outsiders (or Gentiles) until it became forbidden to even be in one of their homes. The strictest Jews wouldn’t “sit on the same couch, nor eat or drink out of the same vessel. The very dust of a heathen city was defiling” (Source).