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I have no experience with gold Monitor Audio, only silver and bronze. In my opinion the older the better. But it's only a personal preference and may not apply to golds. They gradually sounded more hifi.
The GS range is a noticeable difference over the GRs, but wouldn't call it significant. The GX's are significantly better than the GRs IMHO. However, it really depends on how the OP defines "significant".
The GX differes from earlier incarnations because of the ribbon tweeter, whereas the older models had the hard dome tweeter.
I actually used to own a pair of GR20s which I bought brand new and sold 6 months later. An unexpected upgrade opportunity arose and it wasn't because I didn't love them. My main current system is what I upgraded to. These speakers are for my bedroom system. I generally disliked the ribbon speakers so I'm not interested in the GX series.
If the truth be known I actually wanted a pair of Gs60 but they only come up for sale once in a blue moon let alone in the finish that I actually want. Think I'll give the GR20s a miss and keep waiting patiently. Would really love to get a pair of GS60s.