Reading through the
GTS Technical Training doc, I noticed a table that specifies that the GTS has a wider front track by 17mm compared to the the base M4. Further, it is also specified that the GTS wheels have the same offset as the base M4 (ET29 front and ET40 rear), meaning that the wider track would not come from the wheels. Before anyone jumps in about the 0.5" wider wheels from the GTS, note that they do not contribute to a wider track since track is measured from the tire center line. The GTS does seem to have a "flusher" look than the base car IMO. A wider track by 17mm plus the 0.5" wider wheels would push the front wheel face out by ~15mm.
If all this info is correct, that would mean that the GTS' front suspension has been modified extensively enough to provide that wider track. Interesting.