Most gas fireplaces burn liquid propane or natural gas. They are very different and it is important to know what you need before you purchase a fireplace.
What’s known as “Natural Gas” is actually composed of multiple gases: Propane, butane, ethane, and methane. Propane, a part of raw natural gas, is separated from the other gas components during processing at most gas facilities. Natural gas is a utility that’s only available in some areas with underground pipes bringing it to your home. Propane is stored in tanks that need periodic replacing or refilling. Most often residences within city limits are connected to the natural gas pipeline. It is often homes outside city limits that are forced to install propane tanks and have fuel delivered on an as needed basis.
Since natural gas is lighter than propane, if natural gas is released into the atmosphere, it dissipates more quickly than propane. Propane sinks, often making it harder to smell and detect as it settles low to the ground. Natural gas is often touted as being safer as it dissipates more quickly and is often easier to detect if leaking.
Location and available infrastructure are going to be the two biggest factors when deciding which gas you will use. Most residences will have established natural gas piping coming in off the street or a refillable propane tank on their property. Your local utility company can also help you determine which kind of fuel you have access to.