Well, I can't say anything from personal experience as I haven't done this with a gold mini myself, but I can tell you about the varnishes I've used for other minis.
A matte varnish will retain the look of the model as you see it now without the varnish, but it may reduce some of the shine if you're using metallic paints.
If you're using Non-Metallic Metal techniques, a matte varnish would probably be best.
Here's what a lot of people use for all minis - Testors Dullcote
http://www.amazon.com/Dullcote-Spray-Testors-Enamel-Plastic/dp/B0035LOUMU
A gloss varnish will make the model look shiny, which usually is something that most people don't want, but in your case it's different.
It will make it look slick, glossy, almost wet.
Like candy-colored car paint - ever see old cars with bright glossy paintjobs? like that
For a spray enamel, here's what I use - Testors Wet-Look Laquer
http://www.hobbylinc.com/htm/tes/tes1834mt.htm
What you want is probably some of both, and for that a brush-on varnish should be best.
I've used the
GW Gloss Varnish
http://www.games-workshop.com/gws/catalog/productDetail.jsp?catId=cat470011a&prodId=prod930047a
But I'm sure there are other similar brush-on varnishes that will do just as well.
So after applying the Dullcote spray, you just brush on the gloss varnish where you want it, and leave the rest matte.
When I varnish (again, I'm not painting gold minis)
I give one coat of the Wet-Look Laquer, this is because it's much thicker than the Dullcote and is a better protectant.
Then I give one coat of the Dullcote spray to remove the shine
Then I brush on the gloss varnish in places where I want it to be glossy
I hope this helps, and I hope you can get advice from somebody who has worked on gold metallic paints more than I have.