I have several rifles that I (and previous owners) have put a lot of rounds through. Looking at the gas tube interiors, there appear to be thin, longitudinal lines of rust along the interior of the “flutes” where, I’m sure, the piston head has scraped off the original finish (I assume the interiors had been painted). My question is, does anyone here ever actually bother removing that rust (naval jelly?) and/or repainting the interiors?

I don't think the interior is painted since few paints could take the hot gas.
I think they parkerized the interior, at least my Bulgarian AK-74 built from a demilled unfired kit is.

What you're seeing might be built up carbon fouling.

I just clean my gas tube and don't let it rust.
It only takes a minute to run a large bore bottle or tube brush, or a bronze 12 gauge shotgun brush wet with water through a few times then a little lube on a big patch to clean and protect it.

What you're seeing might be rust from corrosive ammo. I found rust in my gas piston tube and I never fired any corrosive - it was actually from the military ammo fired in rifle the kit came from. I just washed/brushed it out with an emulsion of Ballistol:hot water at about 1:10 or richer and that was the end of it. This stuff is in fact the best treatment/cleaner for the AK, followed by straight Ballistol as the CLP.

Big fan of Ballistol myself. Ive cleaned the bejezus out of them and treated them heavily with Ballistol or CLP. Most are not parts-kits rifles but factory semi-autos (Arsenal, SAR2s, etc). That said, always possible a former owner shot corrosive ammo like M67 or 7n6 through them. So, here’s a question...I was shopping around for spare gas tubes (just in case) and a surplus (guy handles mostly Russian and Bulgarian parts) said that AK gas tubes are “consumables” and at some point need replacement....thoughts?

No, I wouldn't agree with that at all. The original factory gas tubes (really, its the gas piston guide tube) are hardened steel, and corrosion or damage are about the only things that can take them out. While they are replaceable (with some fitting) most AK owners never have to do it regardless of round count or age.

My Saiga rusted all over the place when took it out in the pouring rain. The gas tube rusted pretty heavily rather quickly. I sprayed the inside with WD-40, let it sit a few minutes, then ran a 12 gauge brass brush through it a few times and followed it with a patch. It was perfectly clean after that.