SCSS

HTML

<div></div>

SCSS

$fontSize: 16;
@function toRem($val) {
    @return $val / $fontSize * 1.6 + 0rem;
}


:root {
    --gap-l: #{toRem(20)};
}


div {
  height: 100px;
  width: 100px;
  display: inline-block;
  background: #000;
  width: var(--gap-l);
}