How to refuse to continue as gpa ?

The GPA given to you by the principal is for acting on his behalf and representing him on all the occasion that he has authorised you to do on his behalf.  If you do not want to continue to be his GPA you make your intention clear and write back to him asking him to cancel the said deed and relieve you accordingly or you declare in your notice that you are not willing to be his agent anymore.  The court has got nothing to with your private dealings with your principal.  If you do not want to continue your evidence before the court as GPA, you may give a memo to this effect and withdraw yourself from the case even at this stage, your principal has to ratify your evidence or has to give to evidence on his own ignoring his powers he had delegated to you in this regard.