The Central GST Rules are available on the GST Council website. The latest guidelines as of today are available here:
Chapter VI deals with TAX INVOICE, CREDIT AND DEBIT NOTES.
Section 46 begins:
specifically (Color added to draw attention to specific words):
"digital signature of the supplier or his authorised representative."
The definition of supplier is found in the CGST Act 2017, as it appears in the Gazette of India, which is available from the GST Council website link below.
The definition of supplier in section 2(105) is below:
The definition of person in Section 2 (84) of the CGST Act is below:
Lastly, we could not find any specific mention of whether Class 2 or Class 3 signatures have to be used for signing invoices. However, we know that for various tax filings, corporate filings, EPF and other Government filings, Class 2 signatures are used.
Certifying Authorities issue Class 3 Corporate Document Signing Certificates on an HSM only. However, for Class 2 Corporate Document Signing Certificates, HSM is not required, although security precautions are to be taken in their usage.