Chip Antenna -- Two nets to one pad? | Altium Net Tie
The way I solved this was creating a DWG of the traces I wanted.  I then created pads that represent the feed points.

(Just imagine that 6 is the GPS feed, 7 is the GPS GND feed, and 5 is where both of the signals meet.)  1,2,3 are the Wifi feed.  Pads 4 and 4 are the ANT GND.

Originally, I thought I would use the top layer fill, but that ends up creating a PCB trace antenna.  The signals actually need to be traces that end up at pads.

You can see in the schematic symbol, that I created the ANT GND by using two pads.  I think I probably will just make it one.

For the traces, I'll probably connect them using a polygon.  Unfortunately, you can't define a net using the fills, which is why I am just left with pads.

If you select "net tie" when you create the schematic symbol, Altium will let you "short" (ie. you can merge the nets) in the PCB editor.