For companies and LLP, DSC is compulsory for signing GST registration application.
To sign the registration application, you need to first enroll or register your DSC on GST portal.
To register your DSC, click on first menu and select Register/Update DSC.
You need to download Emsigner for updating or signing you registration application with DSC.
Update your DSC on that page and thereafter you can sign the application with your digital signature certificate (DSC).