Mekanism CraftTweaker support has been integrated directly into Mekanism now (link)

Mekanism adds bracket-handler support to define gas -- a special material state differing from forge liquids

ZenScript
 Copy
<gas:oxygen>
<gas:water> *

Noting that <gas:water> is different from <liquid:water>

As of Mekanism 9.7.0 it is now possible to view all registered gases (including those from other mods) via the command /ct gases

It is also possible as of Mekanism 9.7.1 to get a gas stack/bracket handler by string. Use mods.mekanism.MekanismHelper.getGas(string);

ZenScript
 Copy
import mod.mekanism.gas.IGasStack;

var oxygen = <gas:oxygen>.withAmount(500) as IGasStack;
var oxygen2 = <gas:oxygen> * 500;

Like LiquidStacks, IGasStacks also support some special ZenGetters.
You call the ZenGetters using gas.Getter (E.g. <gas:water>.name)

ZenGetter	Description	Return Type
ZenGetter definition	Description Returns the gas' definition	Return Type IGasDefinition
ZenGetter NAME	Description Returns the gas' name	Return Type string
ZenGetter displayName	Description Returns the gas' displayName	Return Type string
ZenGetter amount	Description Returns the gas' amount in millibuckets	Return Type int

You can set the Object's amount (gas volume in Millibuckets) in two ways, which both do exactly the same:

ZenScript
 Copy
var gas_amount_multiply = <gas:water> * 500;
var gas_amount_zenMethod = <gas:water>.withAmount(500);

An IGasDefinition object contains information on a gas.
You can get such an object using gasStack.definition (check the table above)

ZenGetter	Description	Return Type
ZenGetter NAME	Description Returns the referred gas' name	Return Type string
ZenGetter displayName	Description Returns the referred gas' display name	Return Type string

You can multiply a gasDefinition to return a new IGasStack with the given amount in millibuckets:

ZenScript
 Copy
var gas_definition = <gas:water>.definition;
var gas_bucket = gas_definition * 1000;