DFC, the Chief Flight Examiner requires you to check the medical.
If flights were made without a medical during the validity period, the licence would be invalid and hence any PIC flights would be illegal and would not count towards the revalidation. At least, that's my simplistic view.
No medical = invalid licence
Invalid licence = renewal LST!
bose-x, no need for a GST, just the REV06.4 requirements - 'sufficient' differences training to refresh SEP skills, then apply to NPLG for the SSEA Class Rating to be included in the NPPL - you cannot 'add' the Rating yourself.