Its a personal decision, one only you and your family can make. As a parent of two kids with G-tubes, I'd recommend it. G-tubes worked for us! It is easier (emotionally and physically) to do this when children are younger.
When reading the replies from all those who comment of this subject, opinions are so divided. Many of those who are against this surgery are not parents. SO much has come out recently about the health advantages of having higher BMI. With this new data, comes the push for weight gain via G-tubes. It is alarming to some how it is now an encouraged proceedure for CF kids with low BMI, but in my opinion, parents need to do <b><i>whatever</i></b> it takes to give their kid a fighting chance against this nasty disease.
I am convinced my son would have died last year if it was not for the g-tube keeping him fed. It was his only source of calories. It allowed him to keep his weight on while fighting the bacteria and then gain significant weight after the recovery.
Its not an easy thing to decide. Please PM me if you have any questions!