Modified version of the bourbon linear-gradient mixin ... Generated by SassMeister.com.

bourbon_linear-gradient.scss

// ----
// libsass (v3.2.4)
// ----

@import "bourbon/bourbon";
@import "neat/neat";
@mixin linear-gradient2($pos, $g1, $g2: null,
                       $g3: null, $g4: null,
                       $g5: null, $g6: null,
                       $g7: null, $g8: null,
                       $g9: null, $g10: null,
                       $fallback: null) {
  // Detect what type of value exists in $pos
  $pos-type: type-of(nth($pos, 1));
  $pos-spec: null;
  $pos-degree: null;

  // If $pos is missing from mixin, reassign vars and add default position
  @if ($pos-type == color) or (nth($pos, 1) == "transparent")  {
    $g10: $g9; $g9: $g8; $g8: $g7; $g7: $g6; $g6: $g5;
    $g5: $g4; $g4: $g3; $g3: $g2; $g2: $g1; $g1: $pos;
    $pos: null;
  }

  @if $pos {
    $positions: _linear-positions-parser($pos);
    $pos-degree: nth($positions, 1);
    $pos-spec:   nth($positions, 2);
  }

  $gradients: $g1, $g2, $g3, $g4, $g5, $g6, $g7, $g8, $g9, $g10;
  
  // Purge null values from list
  $full: ();
  @each $item in $gradients {
    @if ($item) {
      $full: append($full, $item);
    }
  }
  $full: zip($full);
  
  // Set $g1 as the default fallback color
  $fallback-color: nth($g1, 1);

  // If $fallback is a color use that color as the fallback color
  @if (type-of($fallback) == color) or ($fallback == "transparent") {
    $fallback-color: $fallback;
  }

  background-color: $fallback-color;
  background-image: -webkit-linear-gradient($pos-degree $full); // Safari 5.1+, Chrome
  background-image: unquote("linear-gradient(#{$pos-spec}#{$full})");
}


$medium-purple: purple;
$dark-purple: darken(purple, 20%);
.selector{
  @include linear-gradient2(to top, $medium-purple, $dark-purple);
}

@function color($val) {
  @if type-of($val) == color {
    @return $val;
  } @else {
    @return null
  }
}

.block {
  color: color(#FFF);
}

SassMeister-output.css

html {
  box-sizing: border-box;
}

*,
*::after,
*::before {
  box-sizing: inherit;
}

.selector {
  background-color: purple;
  background-image: -webkit-linear-gradient(bottom, purple, #1a001a);
  background-image: linear-gradient(to top,purple, #1a001a);
}

.block {
  color: #FFF;
}