To access GearCity: 2nd Gear, you must join the GearCity: 2nd Gear branch using the Steam Beta Builds or the GOG Galaxy Branches features.
On Windows and Linux, you'll start GearCity: 2nd Gear by selecting the option at the bottom of the launch window (#1). And then clicking “Play” (#2).
On MacOS, you do not have this option at this time. To play, launch the game with the Steam play button. To revert to GearCity Classic, switch your Steam Beta Branch to “None” in the Beta window drop-down.
Due to polling, GOG support is currently not implemented at this time. Do you use GOG exclusively, and do you own GC there? Let me know, and I'll get it set up. Otherwise, it'll be a few more weeks before it reaches operational status.