How can I eliminate the harmful gas produced from the heating/manipulation of galvanized steel?
Galvanizing by definition is zinc coating. The zinc boils off ( as 1700 F)  is approached and immediately oxidizes to ZnO , a white powder. ZnO has the unusual characteristic of causing zinc chills = brass founders ague = oxide shakes,  IF inhaled while less than one hour old . After that is is pretty harmless and used in various lotions and creams and rubber products.  So avoid the fumes when welding. However , one recovers from "chills" in about 24 hours so not toxic ( I can verify no lasting affects as I had "chills" many decades ago). There is another potential problem ; zinc can cause liquid metal embrittlement of steel . This occurs where the zinc is not exposed to air as if two surfaces are placed together and the edges sealed by welding. I don't know where you think chromium is coming from other than steel made with scrap ( the majority) typically contains about 0.04 % tramp chrome ( and Ni ,and Cu , and Mo). So essentially all steel  welders in the world are exposed to this tramp chrome.