I've opened a StackOverflow question here:
https://stackoverflow.com/questions/48102082/change-the-size-of-the-gutter-of-an-mdc-layout-grid-using-a-sass-mixin

It's not getting many views though so I hope you don't mind me asking here.
What's the correct procedure for changing the gutter size with the SASS Mixins?

Thanks,
Chris.