I understand that you were attempting to export a PCB Antenna design into a Gerber file using the PCB writer, but you can't find the exported file in your current working folder.
Typically, you should be able to see a zip file containing the Gerber files in the same folder of your PCB antenna design files. I am using the ‘pcbAntennaDesigner’ app for exporting, and I exported with the default name "untitled".
Below is a snapshot of the Gerber Export window:
After exporting, a zip file should be created in the current working folder, as shown below. Your previously save designs should be present.
I hope this helps!