The gear indicator is a 7-segment LED display. There is no "bulb", so to speak. Like pontiacman63383 said, the only way to change it would be to do some soldering.
A while back, I was sold on getting one, and traded a couple emails with HealTech about getting a custom green one made. Twice, they said they'd look into parts availability and get back to me. Both times, I never heard from them again. Their lack of correspondence pretty much ruined my desire to get one.
Now that they offer exactly what I was looking for (a green gear indicator w/o the ATRE). I might actually buy one.