I don't want to sound like I am trying to talk you out of it--I'm not, because I know that g-tubes have been lifesavers for many on here. But I'll agree with fr3ak and suggest exhausting every other possible option first, and be careful if you get one.
My fiance resisted a g-tube for as long as he could, but when he was turned down for a lung transplant, low weight was listed among the reasons he was turned down. He was 20 years old, 5'1" and 86 lbs. So, he finally relented and got a g-tube.
His PFTs were in the 20s and he had cepacia, so those factors almost certainly played a role in his extremely poor outcome. He also was not put on IV antibiotics beforehand, and was only on oral antibiotics, which had long lost any real effectiveness for him. I found a book on CF care later on that stated that CFers should be put on IV antibiotics at least 48 hours before the surgery.
After the surgery, he was in a tremendous amount of pain, and couldn't really cough. After about two days of not being able to cough stuff up very well, it turned into the severe lung infection that took his life.
If it worked for you before, it may well work for you again. As you can see, most of the people here (or their children) who have gotten a g-tube have had good results with it. But, there are risks involved which were downplayed to us, so please research it as well as you can to protect yourself if you opt for the g-tube.