[JesseK]

Generally speaking, unless the rules specifically prohibit something, it is usable/doable*.

They've put a chart at the end of Section 8: The Robot to help determine this -- in all actuality, if you follow the steps through the flowchart carefully and objectively, it works quite well. Following the chart, no gearbox is illegal unless it costs more than $400 or it was custom-made for FIRST in previous years' competitions. Whether the gearbox is practical to use is completely up to you... e.g. a 50lb unbreakable gearbox probably isn't worth it to an FRC team, but the AM planetary gearbox is perfect for some subsystems.

*There are grey areas that are outside the intent of the rule...in this case, the AM planetary is not one of those.