Gas turbines run off flammable gases. The main fuels for these are refinery gas, naphtha, biogas, benzene, and random waste gases from oil processing. There are 3 tiers of single block turbines and a multiblock turbine, similar to steam turbines.
Single Block Turbines
There are 3 gas turbines added by Gregtech, providing LV-HV power. Efficiency decreases slightly with higher-tier generators. These turbines use their fuel intelligently- they only produce as much power as needed, and turn off if there's no power draw at all. They don't ever lose fuel over time, don't waste fuel idling, and don't need lubrication.
Basic Gas Turbine
The basic gas turbine is available at LV. It produces 1 amp of LV power (32 EU/t) at 95% efficiency.
Advanced Gas Turbine
The advanced gas turbine is available at MV. It produces 1 amp of MV power (128 EU/t) at 90% efficiency.
Turbo Gas Turbine
The turbo gas turbine is available at HV. It produces 1 amp of HV power (512 EU/t) at 85% efficiency.
Multiblock Gas Turbine
Available at early EV, there is a large gas turbine. It is bigger, more complicated, and more expensive than single block generators, but can get much higher efficiency. It requires a rotor, which determines the efficiency and amount of energy produced per tick. Note that these are capable of wasting fuel, unlike single block turbines- they'll always use exactly their ideal flow amount, even if the extra power is being wasted. Be careful with these- if you make more power than the dynamo hatch can output (I.E. make 2400 EU/t with an EV dynamo hatch) then the turbine will explode. The power produced is based entirely on your choice of rotor, from 150 EU/t with cheap, low-quality material small rotors, to 25,600 EU/t with a huge infinity rotor. Also, make sure that nothing is in the empty space 1 block in front of the front face of the turbine (where the "rotor" is visually showing), otherwise it can cause issues with the multiblock formation.
To maximize the output and fuel efficiency of the Large Turbine, a Fluid Regulator must be installed on the fuel input hatch. Unregulated fuel flow will cause the turbine to produce less EU/t and produce less EU per L of fuel.
Rotors work more intuitively with these than with steam turbines- they simply say how much EU they'll produce per tick. Multiply the EU/t stated on the rotor with the Efficiency stated on the rotor to find the maximum output possible with this rotor. Ensure that the turbine's dynamo hatch is good enough to accept that much output, or the turbine will explode. Note that this is only the maximum output; the actual output is computed from the optimal flow value. Installing a rotor with a greater EU/t rating will not necessarily increase the turbine's output; for increased output, the new rotor must either result in a greater optimal flow value or have a greater Efficiency rating.
The Fluid Regulator on the fuel hatch must be set to a specific optimal value given by the rotor installed and the fuel used by the turbine. To find the optimal value, simply run the turbine for a short while and then use your Scanner on it to find the Optimal Flow value, and use that to set the regulator. Alternately, you can compute the optimal value ahead of time by finding the burn value of the fuel in EU/L, dividing it into the EU/t stated on the rotor, and rounding down. (To find the EU/L burn value of a fuel, look up the fuel cell in NEI and right-click on it, then look for the Gas Turbine Fuels page to see the burn value in EU/cell, and then divide that by 1,000.).
For example, with a rotor rated for 900 EU/t and a fuel (Benzene) that burns for 288 EU/L, the optimal flow value will be 900 / 288 = 3.125 ~= 3 L/t. Note that a rotor rated for 1000 EU/t would also have an optimal flow value of 3 L/t and (with a properly configured Fluid Regulator) would not burn fuel any faster than the 900 EU/t rotor, nor would it produce any more power unless its Efficiency rating is greater.
There are small rotors available at MV, regular sized rotors, available at late HV, large rotors available in EV, and huge rotors available once you've obtained a fusion reactor. Larger rotors process more gas and are generally more efficient, but are more expensive. Rotors are also used in large steam turbines and in large plasma turbines.
Optimal Flow (Gas)
By now, you should know that each type of Large Turbine (Normal, High Pressure, Gas, Plasma) has a specific nominal EU output.
The nominal EU output of gas is the same as the Optimal Steam Flow. This calculated value should be stated on the rotor you plan to use under "Optimal Gas Flow" or "Optimal Steam Flow." This is actually the nominal EU output that this rotor is capable of generating if given the correct amount of gas at the correct rate.
Since this nominal EU output is in EU/t, we will then want to calculate just how much gas we need to feed into the turbine (and at what rate) to obtain our nominal EU output out of our turbine.
Rate of Gas
We want to know how much gas we need to generate the optimal amount of EU/t from our rotor and turbine. The formula we care about to do this calculation is (Optimal Flow) = (Nominal Output) / (Fuel Value)
Fuel Value of Gas
First, we need to know the fuel value of the gas we are using. Let's say we are using Biogas. In GT:NH, 1 cell of biogas gives 40000 EU. We want the EU/L fuel value, so we divide by 1000, since 1 cell = 1000L. 40000/1000 = 40 EU/L. Now that we have a fuel value, we just need to divide this number from our Nominal Output to get the Optimal Gas Flow.
Optimal Flow of Gas
Let's say we are working with a rotor with 96000 L/s optimal steam flow. The nominal output of this rotor is 96000 = 96000 EU/s. Dividing by our fuel value of biogas gives 96000/40 = 2400 L/s or 120 L/t optimal gas flow.
This means that if our Large Gas Turbine is fed 2400 Liters of biogas per second, our rotor (with 96000 L/s optimal steam flow) will generate 96000 EU/s or 4800 EU/t, before efficiency.
Oil products are the main source of gas turbine fuel. Gas turbines can be ran off refinery gas(giving 160,000 EU per cell) or naphtha (giving 320,000 EU per cell). In addition to these, once you've obtained a distillation tower, there are many odd random byproducts of oil cracking that can be put into a gas turbine, to generate a little bit extra power, if they're not used for anything else.
Biogas is an easy-to-make renewable fuel source. 1 cell of methane can be distilled into 3 cells of biogas, 10L of IC2 biomass can be distilled into 16L of biogas (or 3 cells to 8 cells in a distillation tower), or 10L of fermented biomass can be distilled into 18L of biogas. One cell of biogas produces 40,000 EU. Biogas isn't particularly efficient, but is very easy to make. Biogas can also be used to power an IC2 Jetpack, which is less efficient than powering an electric jetpack and using the biogas to produce EU, but one tank of biogas lasts a lot longer than a full electric jetpack, and is easier to refill on the run.
If you want to use gas turbines as your main fuel source, a pyrolyse oven will allow you to do that! This is a fairly complex processing line but produces a lot of fuel. Use a pyrolyse oven to produce wood tar and charcoal, and have several fluid extractors to fluid extract the charcoal, which will give you extra wood tar. After that, put the wood tar into a MV distillery, and distill it into benzene. This method is complex to set up, and uses a lot of wood, but produces a lot of power and is fully renewable, as well as giving you a bunch of ash (from fluid extracting charcoal) which can be turned into fertilizer to grow more wood.
In general, the more energy-dense a particular fuel is, the less you have to feed that gas per second into the Large Turbine. For example, if you chose to use Benzene over Biogas, you would typically end up feeding less Benzene gas per second vs Biogas per second to achieve nominal EU output (assuming you use the same type of rotor in both scenarios).