Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="grid-wrapper">
  <div class="grid-item">one</div>
  <div class="grid-item">two</div>
</div>

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

@import 'susy';

.grid-wrapper {
  width: 1190px;
  margin: 0 auto;
}

.grid-item {
  @include gallery(6 of 12 70px/35px);
  background-color: #ccc;
}

SassMeister-output.css

.grid-wrapper {
  width: 1190px;
  margin: 0 auto;
}

.grid-item {
  width: 47.05882%;
  float: left;
  background-color: #ccc;
}

.grid-item:nth-child(2n + 1) {
  margin-left: 0;
  margin-right: -100%;
  clear: both;
  margin-left: 0;
}

.grid-item:nth-child(2n + 2) {
  margin-left: 52.94118%;
  margin-right: -100%;
  clear: none;
}

SassMeister-rendered.html

<div class="grid-wrapper">
  <div class="grid-item">one</div>
  <div class="grid-item">two</div>
</div>