P.S. This by no means means anything Elon says about terraforming mars is correct. Making mars habitable is very possible, but it's not some weird moral obligation because of birth rates or whatever that idiot is on about, and making mars a billionaire-owned and -run colony seems obviously bad.

Ok made an account specifically to answer this, love your videos btw!
Mars' issue holding atmosphere isn't a gravity problem, it's because it isn't magnetic. magnets create a force field that keep atmospheres safe from solar wind. Putting a really big magnet between mars and the sun would fix this.