But most gas fireplaces are adjusted to the correct gas input when they are built. Many gas fireplaces, especially direct vent fireplaces, are very sensitive to being overfired, and will start producing large amounts of soot and carbon monoxide if the btu input is exceeded.
Some fireplaces have a high/low flame adjustment, which allows users to adjust the size of the flame. These can be adjusted safely.
But other than that adjustment, I would recommend that you not change the BTU input unless you are a skilled gas service repairman.