jQuery.ajax(
    {
        url: formURL,
        type: "POST",
        data: {
            location : location,
            width : width
        },
        success: function() {
            alert('form good');
        },
        error: function() {
            alert('form issue');
        }
    });

class Custom_GateSelector_IndexController extends Mage_Core_Controller_Front_Action
{
    public function indexAction()
    {
        $gate_location = $this->getRequest()->getPost('location');
        $gate_width    = $this->getRequest()->getPost('width');
    }
}

class Custom_GateSelector_Model_Products extends Mage_Catalog_Model_Product
{
  public function getItemsCollection()
  {
      $topStairs = 'yes';
      $gateWidth = 29.00;
      $rootcatID = Mage::app()->getStore()->getRootCategoryId();

      $collection = $this->getCollection()
          ->addAttributeToSelect('*')
          ->addAttributeToFilter('gate_max_width', array('gt' => $gateWidth))
          ->addAttributeToFilter('category_id', array('in' => $rootcatID))
          ->addAttributeToFilter('type_id', array('eq' => 'simple'))
          ->addAttributeToSort('price', 'DESC')
          ->addAttributeToFilter('status', array('eq' => Mage_Catalog_Model_Product_Status::STATUS_ENABLED));

      return $collection;
  }
}

public function getItemsCollection($gateLocation, $gateWidth)
{
    // …
}

$collection = $gsProduct->getItemsCollection();
$collection->addAttributeToFilter('gate_max_width', array('gt' => $gateWidth))

$gate_location = Mage::app()->getRequest()->getPost('location');
$gate_width    = Mage::app()->getRequest()->getPost('width');

Mage::app()->getRequest()->getParam('location');