There are two parts to this.
- You specify the lack of a mask by creating a void in the solder mask layer on your board; from a previous post with this question:
I'm not sure if there is a direct way to do it with the trace properties but what you can do is draw an identical wire on top of the trace you want exposed but change its layer to "tstop" or "bstop" (top/bottom). The STOP layers are used to define areas where the solder mask does not exist. It's typically used in a polygon to unmask a large area but you can use it with individual wires as well.
- You specify the gold plating by using a PCB process that has ENIG (electoless nickel immersion gold) finish. This may be done with a selection while placing your order or as a special manufacturing note to the fab house.