Change the value to something high, like gamma:100.0 or gamma:1000.0 .

If you don't want to mess with text files, you can use a "Fullbright" texture pack. These are popular because they don't require Fabric or Forge to run.

Save the file and restart your game. Night and caves will now be perfectly bright. 2. Resource Pack Method

For more control, such as a toggle key to turn brightness on and off, use a dedicated mod.