You do not need to complete the experience requirements AND a GST.
If you haven't achieved the 'rolling validity' experience requirements (including the dual training flight) before the proposed flight date, then you will need to do have flown a renewal GST in the 3 months before the date of the flight.
What happens on 3 months and 1 day after the date of the GST if you still haven't met 'rolling validity' experience requirements is something we've been asking the CAA (who made the whole cock-up in the first place) to tell us.