Did he use your E-mail Id and password or his own?
If he used his own mail id and password, then you can request him to share password. Once you get password change E-mail ID and mobile number to your own contacts.
If he used your E-mail id and password then you can read retrieving user id and password of GST.
If you accountant is not sharing details you can write an E-mail to GST helpdesk.