Mass Effect has little use for altered gravity since there's no jumping or anything. However I have changed the gravity value in the config file before, and significant departures from 1g make the game unplayable (i.e. makes the Mako undrivable, makes characters take a long time to fall even just a few meters) They would have had to significantly rework and expand the game mechanics to allow for different gravities, and that would have been an unnecessarily large amount of work for that game.

As a visual experience to explore the Martian geography I think it is quite realistic though