The tube is kept clean because the gasses are blasting it everytime. The fouling has no where to go but out the other end, replaced by more light fouling. If you feel you must stick a pipe cleaner down there (I'm not sure why, it doesn't collect debris), I would strongly recommend against any CLP or other product which will only attract fouling and/or leave burnt residue. Such residue would only serve to collect more heat and help cause your gas tube to fail, same with the outside. Upon the first shot or two, that CLP is going to be vaporized and blown right into your receiver, cause even more fouling. Simply put, any debris is going to get blasted out of there with each round you fire, even if it's replaced with new fouling. I have never heard of or seen a gas tube that failed because it needed to be cleaned, in a rifle that shot jacketed centerfire ammo.
The only time I would even think about the prospect of cleaning it is if I was shooting .22LR. Even then, the manufacture of the conversion kits direct the user to clean out the gas tube by shooting some standard pressure 5.56 though it.