Are there gases from the Earth useful to generate electricity besides "natural gas"?
The category "other gases" may not solely include other naturally occurring gases extracted from the Earth. 

As you state natural gas is predominantly methane, extracted from the Earth.

Another type of gas commonly used is petroleum gas, which is prepared by refining petroleum of wet natural gas. Petroleum gas is propane or butane.

Coal gas, which was manufactured from coal and largely consists of hydrogen, carbon monoxide and methane, is no longer manufactured or used.

Another type of gas that could be included in the category is biogas, which is a manufactured gas made from biological waste products, such as: manure, sewage, food waste and green waste. Biogas is predominantly methane.