This is for the black circuit board ADAFRUIT Gemma specfifically, not the arduino gemma.
- Open Arduino IDE (I'm using 2.0.3)
- Open Adafruit IDE -> Preferences
- Add this URL to the Additional boards manager URLs
https://adafruit.github.io/arduino-board-index/package_adafruit_index.json
4. Click the two squares icon at the right of the text box