I want to have 40px as grid gutter and 80px for left-right page padding.
How can I achieve that? bootstrap has hardcoded to divide in half the inner gutter to calculate left and right margins.
If I set $grid-gutter-width=40px in _variables.scss I get 20px for left and right paddings.
I tried changing boostrap\mixins_grid.scss but messed everything
This isn’t a Sage specific question - but your approach of changing $grid-gutter-width is a step in the right direction. If you want the overall page padding to be different than the grid gutter width than you’ll need to do that manually by adding a rule for .row or .container.
If you need more information, I suggest posting on StackOverflow with a bootstrap tag or something like that.