In a combat situation your gas tube cleaning probably consists or running a solvent soaked patch down then followed by a dry patch (if that). However most readers of this post are probably not in a combat type situation, also most readers do not have access to an armorer to replace rusted or broken parts due to neglect. These things are replaced by the owners themselves (ie read OOPE, Out Of Pocket Expense).
So you may wish to ask yourself, why not clean that gas tube while you are cleaning the rifle?
Just because our rifles (AK) are some of the most robust reliable weapons on this Earth does not excuse us from BASIC maintenance.
A simple swab (12 ga shotgun) or even an oversized cut patch works great, I like to use Ed's red then follow with a dry patch. (try to keep the tube as dry as possible).
Every time I read a post about, "Oh it is an AK you can bury it in poop for a year and it will come up firing" just makes me ill.
These rifles deserve to be respected, AK's are like a fine and lovely woman, treat them well and you will have a life full of pleasure, go contrary to instructions and thier will be the devil to pay"!
Borrowed from Brownell's Acraglas instructions.
Anyway, bottom line clean that tube because I may be the guy buying your AK at the next fun show!