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

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<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);

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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	Beschreibung	Rückgabetyp
ZenGetter definition	Beschreibung Returns the gas' definition	Rückgabetyp IGasDefinition
ZenGetter NAME	Beschreibung Returns the gas' name	Rückgabetyp string
ZenGetter displayName	Beschreibung Returns the gas' displayName	Rückgabetyp string
ZenGetter amount	Beschreibung Returns the gas' amount in millibuckets	Rückgabetyp int

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

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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	Beschreibung	Rückgabetyp
ZenGetter NAME	Beschreibung Returns the referred gas' name	Rückgabetyp string
ZenGetter displayName	Beschreibung Returns the referred gas' display name	Rückgabetyp string

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

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