Following is taken from CBIC's user guide. See 3rd page of the linked file:
https://www.cbic.gov.in/resources//htdocs-cbec/gst/goods_transport_service.pdf
"The liability to pay GST devolves on the recipients for supply of services by a goods transport agency (GTA) who has not paid central tax at the rate of 6% ^^, in respect of transportation of goods by road(in terms of notification no. 13/2017-Central Tax (Rate) dated 28.06.2017 (sr.no.1) as amended by notification no. 22/2017-Central Tax (Rate) dated 22.08.2017, if the recipients (located in the taxable territory) belong to the following category" .........(refer types of recipients defined in sec 9(3) of CGST Act)
From the above, it is clear that, if GST is NOT charges at 12% then, it is mandatory for the recipient** to pay GST under reverse charge. Therefore, a supplier (GTA) cannot levy GST at 5% under forward charge.
^^ - it means, 6% CGST + 6% SGST = Total 12% or 12% IGST.
** - recipient means recipient of transportation services who is liable to pay freight and falling in specified list section 9(3).