Thanks SpeedNintendo for the clarification!

I know that gamma is allowed to be at a maximum of 5.0, but there is also the following rule which adds confusion to me:

"You may not read or write to the world or options files outside of Minecraft in any way, except for reading (not writing) from the statistics or advancements file."

Doesn't this mean you can't edit gamma in the options.txt file? Or have I misunderstood the rule? Isn't options.txt an options file outside of Minecraft?

If this is the case, how can I change gamma without editing options.txt? (Not using optifine because of 1.16)