It's critical that Grays have UVA and UVB lighting. I highly recommend this one: https://www.amazon.com/Zoo-Med-Avian-Sun-Bulb/dp/B000KH9S88/ref=sr_1_1?ie=UTF8&qid=1468538019&sr=8-1&keywords=bird+light
I don't know if a plant light would give the proper spectrum or not but I do know this Avian Sun Bulb is inexpensive and really does the trick. It does need to be replaced once a year even if it's not burned out because it loses efficacy.