They're the same shade of green, they react to emotion (courage is just fighting spirit by a different name), they have limitless potential. Not much of a stretch here. Similarly, J-Jewels are also crystalized spiral energy tinted red by the Red Planet's manufacturing processes. Guy's Evoluder form is a human body that has become capable of consciously harnessing spiral power--a prequel to humans as we see them in TTGL--hence the "holding GaoFighGar together by force of will" scene in FINAL.