פירוש על עבודה זרה 111:1

This is stricter than above. According to the earlier halakhah, it was forbidden to tread grapes with Jews who did not observe the rules of purity. According to the later halakhah, it was even forbidden to harvest grapes with them, for doing so was considered aiding a Jew in causing the defilement of the portion of terumah that would have to be separated from the grapes.

But in this matter the law became more lenient. No longer was it forbidden to cause ordinary, non-terumah, produce to become defiled. Therefore, one could glean grapes with non-Jews.