Another File or to another record in a related table of the same file?
If you want to bring up the related record on a layout from a different file, you'll need to use GTRR with the "use external table's layouts" option selected and you'll need to use select Window to bring the window for that file to the front.
To another file, relating to a specific field. So I will need to first select "Perform Script" then add the GTRR?
Yes, and it will not be the only script step in your script. It would also be a good idea to test for the presence of related records before using GTRR to bring them up. If there are no related records, you script silently returns an error code and executing proceeds to the next step in your script--just as though there was no GTRR step in your script.