To power the candle, you can either use a USB cable or a battery pack. Two CR2032 batteries are enough to light it up, thanks to the Gemma's voltage regulator dropping the power to 3V. I haven't done any testing to see how long they'll last; you'll know the batteries are getting low when you start seeing green flickering as the candle dims.
Since I've built several of these and I want them side-by-side, I found a cheap powered USB hub that works very nicely.
To make the candle look real, it will need to be covered by a semi-opaque chimney or globe to diffuse the LED's light. I found some frosted glass chimneys at my local hardware store. They're supposed to be replacements for broken fixtures, but they look great here. Almost anything is possible -- consider a structure covered in tissue paper or a white paper bag or even an old wax candle that's been hollowed out.
In this project, I used one Gemma for each candle, which makes it very easy to run them from batteries and place them anywhere. But there's no reason why one Gemma couldn't run multiple LEDs if they were wired together. This could be done to create a string of candles or even a multi-wick LED candle.
This LED candle only flickers a realistic yellow/orange color, but there's no reason it couldn't have different a colored "flame", or even randomly rotate through colors.
One small caveat: because of the way NeoPixels are designed, their three internal LEDs (red, green and blue) constantly flicker at 400 Hz to hold a color. This is far too fast to be seen with the human eye, so they appear to have a steady, smooth light. But cameras can see the refresh flicker. In still photos, the effect creates "banding" in the photo (visible in the photos above). In videos, the effect creates constantly moving horizontal bands that are very noticable. DotStar LEDs don't have this problem; hopefully Adafruit will come out with through-hole versions of those soon!