Here's why:
* Gentle is a one-syllable adjective.
* For most one-syllable adjectives, we add "-er" to form the comparative (e.g., "smaller," "faster," "nicer").
* If the adjective ends in "-e," we simply add "-r" (e.g., "larger," "more," "brighter").
Therefore, "gentler" is the correct comparative form of "gentle."