If this chassis is going on the road, maybe you want to look farther than that supplier (4130?). Also, if you are going to weld it together, you want to consider the possibilities of killing yourself from the zinc fumes. By the time you weld it up anyway, you will have either had to grind away the zinc before welding or it will vaporize away anyway from the heat and so EVERY joint will be inprotected from the elements anyway. If the supplier doesn't know what kind of metal it is, maybe that's not a good source. At the very least, get a sample of the material and see how it drills and welds. As "galvanized iron", it may not have been made for a stressful use such as your chassis, so they might have used some pretty poor steel to make it. Here's how to find out something about it: Make a fresh cut in the end of a sample that you can put into a vise. Make another sample the same size. Remove the zinc from one of the samples. Plug the other end tightly with a wad of paper or a shop rag. Then heat the zinc-removed end up to a high red heat (almost orange in a dim shop). IMMEDIATELY plunge the heated end into a pail of COLD water and swish it around vigorously until it is cold. ****The reason for plugging the "other" end of the tube is so that steam doesn't come flying out at you when you plunge the other end into the cold water, so don't skip that step!!!!! And don't stand right in back of the tube anyway and you plunge it, just in case! Now put the unheated tube in a vise and do some filing on its freshly cut end. The idea is to note how much difficulty it takes to get the file to cut. Then try the same test with the other piece of tubing, the one you just "quenched". If it requires more filing effort to get the file to cut, then you have steel. If it's exactly the same amount of effort, then you have EXTREMELY low carbon material, or "iron" for all practical purposes.
Why this works: What you have done is to attempt to Harden the sample you heated and quenched in the water. If that sample had no carbon content, then it would not have hardened at all and so there'd be no difference between the two samples. Your material is probably a low carbon steel, in the 0.15 to 0.3% carbon range, generally held to be non-hardenable for industrial applications, but that doesn't mean it won't harden at all. So, if there's any significant amount of carbon in it, it will harden up some by the method described above. That will make it harder to file than the unheated or quenched sample.
"Iron", as another poster said, is pretty hard to make these days, since a lot of what we use is made from recycled automobiles. Very low carbon steels (Iron plus 0.08% Carbon, called "1008") are used quite frequently in industries where a lot of stretching ("deep drawing" for example) will be applied. For almost all practical purposes, this material could be called "Iron". Us blacksmiths like it for doing higly detailed forming, chisel and filing work because it doesn't crack or split or require annealing very often.
for an example of such work if you are interested.
I think "galvanized iron" is just an old term still applied to stuff like what you see at your supplier, just as people talk about "wrought iron railings" when they are mostly made from A36, the low carbon (0.15-0.29% carbon) steel you'd usually get if you just walked into a steel yard and asked for a "piece of steel".
Galvanized pipe is made from steel of unknown parentage. But presume it is real junk. These days, the 'iron' part of it refers to the sizes base on the old pipe system where it is the ID that is important. The only real specs it needs to meet is to be round and hold 300 PSI. The galvanized coating on it needs to be cleaned off to avoid getting the zinc into the weld and into your lungs. This is NOT the sort of thing you want to use for any kind of mobile chassis.
The tubing you want comes in 3 general classifications:
-cold or hot rolled welded tube. Typically around 1008 to 1018 mild steel, available in many wall thicknesses, outside diameter in reasonable fractions of an inch ie 1-1/8" or 1-1/2" Yield strengths vary but 50,000 psi is a reasonable number to start with.
-DOM (Drawn Over Mandrel)steel tubing is the same basic tube as above except that it has been drawn over a mandrel to make it more uniform and work harden it. Typically made from 1018 or 1026 steel. The work hardening moves the yield up to perhaps 80kpsi
-4130 alloy (and some similar grades) This can be either welded or seamless, runs around 90Kpsi yield if it is annealed, can be pushed to over 200kpsi with heat treating.
Prices on the last 2 will be enough to take your breath away. But for that price you will get uniform product, known specs, and mill certification sheets to prove it.
I couldn't tell you which one you need to use without a lot more detail. If this is for any kind of sanctioned racing, the tech inspectors will have a lot to say about what you use. For our rules, we have to use 1026 DOM in certain specified diameters and wall thicknesses, with a mill cert sheet handy to show the inspector. Then they use an ultrasonic thickness gage to prove we are not lying.