This past February I rented a condo on Maui (not for from Lahaina). I had a great experience and was able to find two 100% gluten free places to eat on the island (one a restaurant and the other a bakery). I usually mitigate all risk by not eating out, but felt comfortable enough with these two places to give them try. I also had no problem finding gluten-free foods at the supermarkets.
The Maui Sugar shop Organic/gluten free Bakery was an awesome find and the owners are some awesome folks. It is in Lahainia and hard to find (I stumbled upon it because I took a wrong turn trying to find a parking spot). Most shops will actually be on Front street, but this little gem is tucked in a parking lot (if you find Panama Jacks on Front street...walk to the parking lot in back and it will be a few storefronts down in the same building as Panama Jacks back door. The treats are fantastic and so are the owners (I met each of them on separate visits and both are awesome folks). I only visited twice as I found them at the end of my vacation. I would have stopped there every day if I found them earlier. They are doing it right and earned my business (you just want folks like that to succeed).
I also went to the Maui Brick Oven in Kihei. They are open for dinner. Don't let the strip mall location turn you away. Stop in and give them a try. You may find yourself grabbing a pizza to go too.
There are plenty of gluten free groceries to be had in stores. There is a wholefoods in Kahului and the Safeway in Lahaina that had enough gluten free items to be fine (there was another grocery store closer to my condo that also had gluten-free food but not a as much(can't remember the name right now).
Food can be expensive, but if you shop at safeway sign up for there rewards card (I think that is what they called it) and you'll get some good discounts. They also stock red-bridge if you want a gluten-free beer.
If you have any questions let me know!