Unless your gas is pure hydrogen, you still have sufficient carbon to affect the metal unless you control things very carefully. (Carbon is extremely soluble in iron.) Gas is used, natural/propane etc., for some purposes, but can't attain the temperatures you can get with charcoal or coke, and it's reportedly much more difficult to achieve a large enough area with uniform high temperature with gas. Gas is okay for fitting horseshoes, perhaps, but any major smithing is probably best done with a more traditional (solid) fuel. (Or so I've been told by people who should know.)

I have seen a couple of "performance smiths" who set up at festivals to show off how they make the stuff they have for sale and who use portable gas forges for the demonstrations. Maybe next time one of them is around I'll get a chance to ask if he does all his work with gas... but I'd be surprised if he does. The stuff done on site at the festivals is generally pretty simple.

John