normal boiler + 2 steam engines: 100% power, 30 pollution/s, 100% fuel use
gas boiler + 3 steam engines: 150% power, 30 pollution/s, 150% fuel use
gas boiler + 1.5 steam turbines: 250% power, 30 pollution/s, 150% fuel use

This means that gas boilers pollute less per unit of power and gas boilers + turbine gain a significant fuel efficiency bonus.

This makes the gas boiler a fluid-burning chimera somewhere between normal boilers, heating towers and nuclear. Needless to say, fluids are harder to get than solid fuels, so you're going to need plenty of oil and a good refinery setup.

A note on the different fuels you can feed to the gas boiler by default:
Crude Oil:1MJ per unit (1.67MJ w/turbines), 150% polluition
Heavy Oil:1.5MJ per unit (2.5MJ w/turbines), 150% polluition
Light Oil : 1.2MJ per unit (2MJ w/turbines), 100% pollution
Petroleum: 0.9MJ per unit (1.5MJ wi/turbines), 50% pollution

This makes Heavy Oil your best fuel when it comes to efficiency and Petroleum your best fuel when it comes to pollution per unit of energy. If possible, avoid burning crude oil, you'll get a lot more out of refining it.

By default, the gas boiler and steam turbines are unlocked by the flammables research. (Steam turbines also unlock with calcite processing)

This mod depends on and overrides the Superheat Gas Boiler. and depends on Fuel Customizer.

I recommend some alternative sources of mid-game oil like Coal Oil and Cargo Ships: Oil Rig.