equal height grid columns using susy and flexbox

gistfile1.scss

@import "susy";

$total-columns  : 7;
$column-width   : 4em;
$gutter-width   : 1em;
$grid-padding   : $gutter-width;
$break          : 12;

@mixin flex-span-columns($columns, $context: $total-columns, $padding: false, $from: $from-direction, $style: fix-static-misalignment()){
  @include span-columns($columns, $context, $padding, $from, $style);
  flex: 0 0 columns($columns);
}

body {
  @include container($total-columns, $break);
}

ul {
  @include at-breakpoint($break){
    display: flex;
    flex-direction: row;

    &.multiple-rows {
      flex-wrap: wrap;
    }
  }

  li {
    @include at-breakpoint($break){
      @include flex-span-columns(4);
      
      &:nth-child(3n){
        @include omega;
      }
    
      &.large {
        @include at-breakpoint($break){
          @include flex-span-columns(8);
        }
      }
    }
  }
}

what is $grid-padding for?