From what I can see here the gas fireplace inside the house appears to vent through the slab and out side of the house. Would there be some sort of a fan assist to help the exhaust?
It may be combustion air on a ventless unit although the ones I’ve seen just recommend cracking a window…
Look up the make and model on Google to get the installation installation instructions and verify the installation. We can’t see much in your picture to be of much help.
If it is direct vented the vent pipe structure is much like a “B” vent. An inner pipe exhausts, the outer pipe brings in fresh air for combustion. If there is a glass front and the logs and flame are enclosed then it is a direct vented unit. If there isn’t a front and it is just gas logs it could be a vent free system or gas log system that uses the inside air. Which requires a cracked open window, supplemental fresh air, or if the house is old and not air tight, combustion air may not be an issue.
I have installed and serviced hundreds of gas fireplaces. Do you have multiple pictures?
FYI - Vent free systems dump a ton of moisture into the home. Which can migrate and condense on the cold side of walls and ceilings.
And it is supposed to drain back to the furnace (see furnace manufacturers installation instructions), so if your pic is of a vent that goes down from the furnace before going out through the slab and back up again, it probably is not installed correctly.