|Hoptwoit's reference to STD 571, Appendix A is appropriate, and useful, however the installation of a G meter would slip between the cracks in it. For myself, I would not tend to find that I need to issue an STC for the replacement of one G meter with another, if they were both MIL Spec or TSO'd, and the aircraft parts catalog said "or equivalent". I'm not certain that a VFR Pitts would require TSO'd equipment, as long as the proposed instrument was AN, MIL Spec, or "equivalent". G meters do have a special niche in this though, as they do indicate a value which is subject to limitation in the aircraft type design, so it should be "equivalent", and airworthy. I would be surprised if an inspector or AME would have issue with an airworthy G meter change within the meaning of "equivalent". But that determination would be their job, not mine. I only issue STC's if requested to do so.
I have once seen a digital electric G meter. It was installed in a DA-42 I was flight testing, for the zero G testing we had to do. It seemed to work well, though I'm not sure if it would be "approved" for a certified installation. If it were to be installed in the electrical system, it would not be a major mod in and of itself for that - nor any other installation after a bus circuit breaker.
571 Appendix A says:
"(7) alter an electrical generation device, or the electrical distribution system between the generating source and either its primary distribution bus, or any other bus designated as an essential bus? "
you see, in electrical terms, you have altered the electrical system between the generating source and the primary bus. That is without a doubt...whether your inspector knows that or not.
Anything you would install between the generator/alternator and the primary bus would be carrying the entire, or at least a very large, electrical load. The only "Instrument" I can think of which could carry this load would be an ammeter (shunt). That would be an alteration, and require "approved" data = STC, which is commonly available with the instrument for such installation. An MS/AN G meter is not that - it's not even electric - it's not connected to anything in the plane!
Otherwise, from the primary bus, there are generally circuit breakers, to which the various circuits are connected and thus protected. Anything you install to the "load" side of a circuit breaker is not between the generator and primary bus, it's after the primary bus, and therefore a "no" on the Appendix A "test" for major - in terms of electrical.
CID points to an absence of "acceptable" data, and I agree that AC43.13 makes no mention of G meters, so is not a source of acceptable data.
" “acceptable data” - includes:
(a) drawings and methods recommended by the manufacturer of the aircraft, component, or appliance;
(b) Transport Canada advisory documents; and,
(c) advisory documents issued by foreign airworthiness authorities with whom Canada has entered into airworthiness agreements or memoranda of understanding such as current issues of Advisory Circular 43.13-1 and -2 issued by the FAA, Civil Aviation Information Publications (CAIPs) issued by the Civil Aviation Authority (CAA) of the United Kingdom, or Advisory Circular, Joint (ACJs) issued by the Joint Aviation Authority (JAA) or Acceptable Means of Compliance (AMC) issued by the European Aviation Safety Agency (EASA); and,
(amended 2009/12/01; previous version)
(d) drawings and methods found appropriate by a delegate in conformity with paragraph 4.2(o) and subsection 4.3(1) of the Aeronautics Act. (données acceptables)
(amended 2009/12/01; previous version) "
If (a) items are not available, (d) might be an option - the OP knows a delegate.
There's been some mis-information posted in this thread, so read with caution. The fact that something can be written in a post, does not make it authoritative. Always follow a lead back to the authoritative data to make your final determination.