Here's why:
* Superlatives are used to compare three or more things and indicate the highest degree of a quality. They often end in "-est" (e.g., gentlest) or use "most" before the adjective (e.g., most gentle).
* Gentle is a positive adjective, meaning it describes a positive quality. It doesn't compare it to other things.
To make "gentle" a superlative, you would use either "gentlest" or "most gentle."