[Peter Serio]

You don't say what year this set is but if you post some decent photos of it, (front and back) I might be able to tell you if it's real or not.

The speed-warning housings are not the same a a regular housing (non-rally gauges) and there are some unique parts on the backside of the gauge cluster that make it work.
I was always under the impression that you had a choice of: regular gauges, rally gauges as an extra cost option and speed-warning (also extra cost) and that came with a little red warning lamp that would light up after you got to just below around 1/4th of a tank of gas. The fuel gauge with the low fuel warning lamp is also different than a normal fuel gauge in a set of non-rally gauges.

It may depend on the year. I have a set of '67 speed warning gauges here and that cluster has the low fuel warning lamp.

I saw a set of '68 GTO rally gauges for sale on ebay (as of last week) which appears to be a combination of 2 sets of gauges. Rally and speed warning combined into one set. I do not believe that those were ever made like that from Pontiac.