Which Melachas are forbidden to be done during Chol Hamoed?
Whether one learns the prohibition of Melacha on Chol Hamoed is of Rabbinical or Biblical origin, all agree that the definition of which Melachas are forbidden and which are permitted were handed over to the discretion of the Sages. Hence, all those Melachas which are forbidden to be done on Shabbos and Yom Tov are likewise possibly forbidden to be done on Chol Hamoed, based on the rules given to us by the Sages. In general, the Sages only prohibited those Melachas which involve effort and are time consuming. Even regarding these Melachas, in certain instances the Sages allowed for even these Melachas to be performed.
May one do Melacha for the sake of a gentile? No. Those Melachas that are permitted for a Jew to do for himself are forbidden to be done even for the sake of a Gentile.
 Kitzur 104/5