Where exactly is that file? I couldn’t find it in the location (/wp-content/themes/gather/gather/vc_templates) and in case I was in the wrong one, I couldn’t find it either in the whole folder I downloaded on Wednesday 13. I’m attaching pictures.
First of all I have to notice you that theme template files and vc_templates folder should be located in \wp-content/themes/gather/ folder ( not in gather sub directory like yours: http://prntscr.com/btzkwv )
And the file to edit (add your api key ) is \wp-content/themes/gather/functions.php file.