AC Expansion Valve
 

...for a GSL-SE. Where is it located? Are they difficult to change/expensive to replace?

It is located in the vehicle on the passengerside inside the A/C box where the 2 A/C hoses come thru the firewall.
The procedure can be found by downloading the FSM per member trochoids link.
As far as cost goes of part. www.rockauto is a good source.