There seems to be claims in both directions, but Mark did found one explanation that seemed to reconcile these different views:
viewtopic.php?f=8&t=11236
In short, these two machines were developed in parallel: the RX-79[G] was produced using the leftover complements from the RX-78 development while the RGM-79[G] was built in the same production lines for gathering data for later GM models. These units are essentially siblings, not quite compatible with any other EF model (leaving aside some even more rare prototypes, such as the Blue Destiny units) and whose development mainly obeyed the EFs need for stepgap units that allowed them to stop Zeon’s advances while development of mass production models bore fruit.
The Gundam [G] has slightly higher performance thanks to the Gundam parts, but since the components of the GM [G] are more readily available you can expect that in the long run all surviving [G] units would end up with performance closer to the later, as the prototype components break down and need to be replaced with available parts.