_getGutterSize(containerWidth) {
    let size;
    if (typeof this.options.gutterWidth === 'function') {
      size = this.options.gutterWidth(containerWidth);
    } else if (this.options.sizer) {
      size = getNumberStyle(this.options.sizer, 'marginLeft');
    } else {
      size = this.options.gutterWidth;
    }

    return size;
  }
Returns the gutter size, based on gutter width and sizer options.
@param {number} containerWidth Size of the parent container.
@return {number}
@private