Most dictionaries list "gentler" as the comparative form of "gentle", but "more gentle" also sounds correct. According to this forum post, it is.
It is not incorrect, though the form isn't just frequently used.
Examples from this article:
A: Did you say it was less hot in there?
B: No, I said it was more hot.
There’s nothing more dull than having to listen to him.
More examples from another article:
Here are the conventional rules for forming the comparative form of a one-syllable adjective in English.
Now for the exceptions to the rule.
Sometimes using more instead of -er with a one-syllable adjective is an acceptable stylistic choice:
The writer wishes to emphasize the comparison. “He promised to paint the chair pink, but when the paint dried, it was more red than pink.”
The one-syllable adjective occurs with an adjective of two or more syllables. “The lecture was more dull and lengthy than the previous one.”
It is easier for the speaker to say. “Both views may be right, but mine is more right than yours.” (Other one-syllable words that compare with more are real and wrong.)
It is of some note that this article says "conventional" and "stylistic", which supports that even other combinations of a "more/less" and a monosyllabic adjective not falling within the scope of the exceptions listed above are not grammatically incorrect though they are usually avoided or advised against, and it is no more than a matter of convention and style.