In the old days, sometimes after a long time. This was generally due to the poor quality of galvanizing. Today using galvanized pipe generally presents no threat. Plumbers tend to use black pipe with gas for two main reasons. One is that it costs less than galvanized (except I’ve seen some home centers charging more for black which must be because of consumers lack of knowledge of “cost”). The other is that if galvanized pipes are used for water then using black for gas distinguishes the two uses. Imagine using galvanized for both gas and water in the same house? That could present a problem in people distinguishing which pipes are carrying what. I like to suggest painting galvanized pipes used for gas yellow. Today there exists flexible stainless steel gas pipe and it always yellow (that I’ve ever seen) so painting piping that is for gas yellow seems like a good idea. If pipe is in the ground it will tend to corrode faster (depends on the soil conditions). Many areas do not allow galvanized pipe underground for gas piping and factory coated steel pipe must be used. In some areas plastic pipe is allowed underground (with electric wire above it to allow for locating of that plastic gas pipe as well as warning tape above that pipe as well).