Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="gallery-item">
</div>
<div class="gallery-item">
</div>
<div class="gallery-item">
</div>

SassMeister-input.scss

// ----
// Sass (v3.4.7)
// Compass (v1.0.1)
// Susy (v2.1.3)
// ----

@import "susy";

// Returns $list as a string
// -------------------------------------------------------------------------------
// @documentation http://sassylists.com/documentation/#debug
// -------------------------------------------------------------------------------
// @example debug(a b c d e)   => [ a, b, c, d, e ]
// @example debug(a b (c d e)) => [ a, b, [ c, d, e ] ]
// -------------------------------------------------------------------------------
// @param $list [List] : list
// @param $pre [Boolean] : enable/disable variables type and proper indentation
// @param $level [Number] : internal variable for recursivity
// -------------------------------------------------------------------------------
// @return [String]

@function debug($list, $pre: false, $level: 1) {
    $tab: "    ";
    $indent: "";
    $break: if($pre, "\A ", "");
  
    @if length($list) == 0 {
        @return "( )";
    }
  
    @if length($list) == 1 {
        @return if($pre,  "(" + type-of($list) + ") ", "") + $list;
    }

    @for $i from 1 to $level {
        $indent: $indent + $tab;
    }

    $result: "[" + $break;

    @for $i from 1 through length($list) {
        $item: nth($list, $i);
        $result: $result + if($pre, $indent + $tab, " ");

        @if length($item) > 1 {
            $result: $result 
                + if($pre, "(list: " + length($item) + ") ", "") 
                + debug($item, $pre, $level + 1);
        }

        @else {
            @if $pre {
                $result: $result + "(" + type-of($item) + ") ";
            }
      
            @if length($item) == 0 {
                $result: $result + "( )";
            }
      
            @else if type-of($item) == string {
                $result: $result + quote($item);
            }
      
            @else if $item == null {
                $result: $result + "null";
            }
      
            @else {
                $result: $result + $item;
            }
        }
    
        @if $i != length($list) {
            $result: $result + "," + $break;
        }
    }

    $result: $result + $break + if($pre, if($level > 1, $indent, ""), " ") + "]";

    @return quote($result);
}

// Mixin displaying clean debug
// -------------------------------------------------------------------------------
// @param $list [List] : list

@mixin debug($list) {
    body:before {
        content: debug($list, true)              !important;

        display: block                            !important;
        margin: 1em                               !important;
        padding: .5em                             !important; 

        background: #EFEFEF                       !important;
        border: 1px solid #DDD                    !important;
        border-radius: .2em                       !important;

        color: #333                               !important;
        font: .75em/1.5 "Courier New", monospace  !important;
        text-shadow: 0 1px white                  !important;
        white-space: pre-wrap                     !important;
    }
}

$span-test: 3 of 12 no-gutter;

/**
 * Simple debug function
 */
body:after {
  display: block;
  content: debug(parse-span($span-test));
}

.gallery-item {
  height: 100px;
  background: red;
  @include gallery($span-test);
}

SassMeister-output.css

/**
 * Simple debug function
 */
body:after {
  display: block;
  content: "[ [ span, 3 ], [ gutter-override, no-gutter ], [ columns, 12 ] ]";
}

.gallery-item {
  height: 100px;
  background: red;
  width: 23.7288135593%;
  float: left;
}
.gallery-item:nth-child(4n + 1) {
  margin-left: 0;
  margin-right: -100%;
  clear: both;
  margin-left: 0;
}
.gallery-item:nth-child(4n + 2) {
  margin-left: 25.4237288136%;
  margin-right: -100%;
  clear: none;
}
.gallery-item:nth-child(4n + 3) {
  margin-left: 50.8474576271%;
  margin-right: -100%;
  clear: none;
}
.gallery-item:nth-child(4n + 4) {
  margin-left: 76.2711864407%;
  margin-right: -100%;
  clear: none;
}

SassMeister-rendered.html

<div class="gallery-item">
</div>
<div class="gallery-item">
</div>
<div class="gallery-item">
</div>