Mekanism

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

<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);

Example

import mod.mekanism.gas.IGasStack;

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

ZenGetters

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

Setting the Object’s Amount

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

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

IGasDefinition

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

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

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