The LED strand has 3 wires. The red-striped wire goes to the Gemma's Vout pin, the middle wire goes to D1, and the third wire goes to G.
Be sure you are wiring to the IN end of the strip. LED strips have an IN and OUT end and they won't work if you wire them the wrong way. If you loo closely at the back of the pixels you can make out an arrow pointing in the direction of data flow (from in to out). This is really hard to see, so it's usually reliable to start from the end of the strip that has the male connector. If you find your project isn't working, you may have a strip that's manufactured differently -- try starting at the other end.