Explosive atmospheres: mixture with air, under atmospheric conditions, flammable substances in the form of a gas, vapour, dust fibers, or flyings which, after ignition, permits self-sustaining propagation.

The definitions found in IEC 60079-10-1:flammable gas and vapour atmospheres.
Zone 0 = an area in which an explosive gas atmosphere is continously present, for long periods.
occurence of explosive atmosphere for more than 1000h/yr
Your gas tank would fit this definition.

The internal of the holder will be Zone 2.

There will also and more importantly be an external Zone 2, the extent of which can vary and will depend on a number of factors:

Dry or water sealed holder?
Diameter of the water sealed holder
Indoor or outdoor location.

Is this something like bio gas or methane storage?

Based on this info, I would suggest Zone 2 internally (start up and emptying/shutdown operations), Zone 2 at interstitial space (release/rupture from inner membrane) and Zone 2 around any flange or breakable connection in the pipe line feeding into or taking gas from the gas holder.