Heh, I'm an Air Traffic Controller, tower controller, I like to look out the window, but the TRACON/Center guys get paid $$$ more.
The GPS you are talking about is called ADS-B (Automatic dependent surveillance – broadcast), it hasn't been fully implemented yet. "The FAA has published a rule requiring ADS-B transmitters in many types of airspace (ADS-B Out) to take effect on January 1, 2020, but there is no mandate for ADS-B In, which receives data and provides it to in-cockpit displays." -- I work Class Delta airspace which has no rules for ADS-B at this time.
Stuckmic.com is the forum where air traffic controllers chat. Mostly just people waiting to get hired BSing with each other because it is a 2 to 3 year hiring process, some people talk about Union issues, and others have questions about specific facilities. We've got a rocky road ahead with a new man in charge though...
As to the OP's question to WHY there is no ATC frequency monitoring app for Mac? Simply because none has been written. It's a niche market, and there are easy alternatives. The example liveatc.net was provided, it takes literally seconds to dial in any ATC frequency online. You can even download a .pls file from liveatc.net and it will load up in iTunes to stream at your leisure. Seems like a good opportunity for someone to do a college coding project!