I made small adjustments, switching out the Gemma board for a Trinket, which is what I had available. Think of the Trinket and the Gemma as tiny Arduinos, sort of. They aren't equipped with as many pins, and actually work a bit differently. For example, with an Arduino Uno, I can connect via the serial USB, send data to the board and receive data back. The Trinket doesn't allow for serial connection over the USB, and requires something called a Bootloader. If you'd like to learn more, see this Overview of Trinket from Adafruit's Learning System.
While there is a slight learning curve involved with getting Trinket to talk with the Arduino IDE, it was worth the time. The big advantages to Trinket (and Gemma) are size and cost. As you will see in the photos below, the Trinket is very tiny, and as of this writing, costs a mere $8.
I won't go over the tutorial, as that is available from the link above. I will say that it took some trial and error to get the soldering just the way it needed to be, and in the end I used a larger battery pack than what was stated as I knew the Trinket had a voltage regulator and could handle the extra input. Overall, a fun project!