To Abhishekabs,
Gold is not good even for DC because it is almost 40% less conductive than copper. Gold plating on RF board without solder mask is actually may kill the board performance because if you try to solder something, for example end-launch, solder will immediately go around the trace and change the metal thickness, roughness and replace the external layer. It will change line impedance and increase loss. Plating gold under solder mask is meaningless. The only reason to use gold is when you need to keep exposed contact in good shape for a long time. This is why gold used for some of the connectors, but not for all and not for very high frequency. SMA is not very high frequency and high quality connector.
Regarding frequency dependence it is skin effect. For example at 4.3 GHz the 90% of electrical current will flow through the outer 1 micron of the metal. If it is gold plated loss will be significantly higher compare to copper. Microwave lines must be cold rolled copper in order to minimize the loss.