When searching for flights from southwest.com, you receive an error 504 page with a gateway_error.

ProxySG or ASG

Adding southwest.com to your SSL interception exemptions list is the current workaround for this issue.

Alternately, the following code can be used to maintain SSL Interception:
<Proxy>
url.domain=//southwest.com/ action.Remove_X-Bluecoat-Via(yes)

define action Remove_X-Bluecoat-Via
delete( request.header.X-BlueCoat-Via )
end

If VPM is preferred, create a new rule or use an existing rule that suppresses this header in a Web Access layer, and set the destination to southwest.com, and set the action to Suppress Headers and choose X-BlueCoat-Via from the list.