Just found a pic on their facebook page of a barrier/match/translator coach no 6397, with GSA painted below the number.
https://www.facebook.com/1965401171...6540117115590/950352378401023/?type=3&theater
(you might have to click and move it round, as it's a panorama)
Also on departmentals.com at
http://www.departmentals.com/photo/6397-2
where it's also a GSA and described as an HST barrier coach
I've also found references to TOPS QSA = EMU translator van, so maybe GSA has been pressed into service as the TOPS code for HST translator coach.
The Q-series coded coaches were usually numbered in the departmental series (e.g. 975xxx, 977xxx), but as this one is numbered in the "normal" coaching stock series, perhaps the G-series TOPS is logical.