FWIW, the downloaded file is named Utah ATV Trails 2016-12345196.gmap-653.tgz and it is 476,160 bytes
It opens automatically in Archive Utility on my Mac, but I might have manually configured it to do that. If this doesn't happen, right-click (or control-click) the file and choose Open With > Other. Then browse to find the Archive Utility program which is located in the /System/Library/CoreServices/ folder on Mountain Lion.
If that works but nothing happens when you double-click the .gmap file, then I would re-install Basecamp. Have seen reports that the app store version sometimes does not work properly, so I suggest that you download directly from Garmin using the link I posted earlier.