Is the MHA Gazette notification of July 31, 2019, therefore the start of the Implementation of an all India NRC?

The Preparation of NRC has been specified (as mentioned above) in Rule 4 of Citizenship (Registration of Citizens and Issue of National Identity Cards) Rules, 2003. The initiation of NRC is specified in Rule 6 of these Citizenship Rules.  So far, specifications under Rule 6 have not been issued.   However, since the preparation of the National Population Register is the required basis for an All India NRC, and given widespread statements by politically influential functionaries, it would be prudent to understand that the present dispensation is laying the basis for the preparation of an all India NRC.