Can you use regular Gain detergent in a he washing machine?

Yes you can. However, by using it you wear out your bearings and washing machine drum so it is actually cheaper if you use the HE detergent because you want your washing machine to last as many years as it can. Buying a new washing machine or regularly fixing the old one is more expensive than buying the specialized HE detergent which can be a bit more pricey that the regular detergent.