Blue GS runs off the Goal Line, behind the post to gain a better position, and retakes a position on the court back in the Goal Circle. The GK follows the GS (taking the exact same route!) and once back on court the GK retakes her position to mark the GS.
Yes the GK is permitted to leave the court to re-take a better position as per rule 8.3.2 (ii) as is the privilege for any player on court.
If the GK attempted to defend the GS while out of court this would need penalising as per rule 11.3.2 (i) and/or 11.2 (ii)