Our company sends out a wide variety of parts for heat treatment. The heat treat suppliers all use natural gas for a fuel source (except in vacuum furnaces, which are electrically heated). By modifying the air/fuel mix, they can alternately reduce, increase, or maintain the carbon level in plain steels. In most cases, they want to leave the carbon as specified so the flame is neutral but for example, in a case hardening application, they will increase the carbon and add other gases to the mix (ammonia is one type) to impart a hard case to a prescribed depth. This industrial processing is very repeatable but the average blacksmith cannot control his fire so closely, so most get some level of decarburization or scaling while forging. To answer your question, a gas forge would actually be easier to control than coal or coke if you pay attention to the fuel mix because it would not tend to change much, once set.
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