Methane Gas Canister
Methane Gas Canisters are used to transport Methane Gas between machines - useful if you do not have any mod with fluid pipes installed. They are made by filling an Empty Canister in a Methane Synthesizer machine which has produced some Methane Gas in its output tank.
The canister can contain a maximum of 1 bucket (1000 mB) of Methane Gas. Vanilla buckets and other liquid receptacles cannot be used to hold Methane Gas, as it is compressed form of the gas and needs special handling in a canister.
The colored bar icon drawn on the side of the canister shows approximately how full it is. There is also a tooltip which states the contents exactly.
If you have other mods installed with fluid-handling capabilities (for example, Buildcraft, IC2 or Railcraft) it's also possible to pipe Methane Gas out of the Gas Liquefier machine into tanks, and then fill Methane Gas Canisters from those tanks in the same way that you will normally fill a bucket.
(It is possible a future version of Galacticraft will add other uses for Methane Gas as a different type of fuel, but there are no current plans.)
One full Methane Gas Canister holds 1000 mB of the specially compressed version of the gas, which is enough to produce 500 mB of Fuel.