Why is the appearance of a gas not always a sign of a chemical change

When you boil water, it simply evaporates. It's still water, just in a gaseous form. If a substance only vaporizes/evaporates that's not a chemical change, it's a physical one. If you calculate that a reaction should produce a gas, and the chemicals aren't at a temperature or pressure that would cause them to vaporize, then you can conclude it was a chemical change.