Is it incorrect to use "more gentle"?

Most dictionaries list "gentler" as the comparative form of "gentle", but "more gentle" also sounds correct. According to this forum post, it is.

EDIT March 14, 2020

Another question investigates the general rules for forming disyllabic comparatives: Conundrum: "cleverer" or "more clever", "simpler" or "more simple" etc. The conclusion is that, outside of words with some specific endings, a dictionary must be consulted for each case. In contrast to that question, this question addresses a specific word and the degree to which modern dictionaries are correct in defining its comparative.

An interesting (to me) side issue in the "more gentle" versus "gentler" discussion involves the potential for ambiguity. The phrase "more gentle" can have two meanings: "gentle relative to X" (where X is the object or standard of comparison) or "additional gentle." The word gentler has only the first meaning. So, at the outset, it should be obvious that gentler is an unambiguous option when you're trying to say "gentle relative to X" and that it is not an option at all when you're trying to say "additional gentle."

In situations where the intended meaning is "gentle relative to X," the broader context usually provides sufficient information to ensure that hearers or readers can quickly surmise the correct meaning. Moreover, in speech, I have noticed that many people tend to stress different syllables when using "more gentle" in the sense of "additional gentle" than when using it in the sense of "gentle relative to X." Consider this potentially ambiguous sentence:

Patients who suffer complications from the initial infection require more gentle treatment.

On its face, the sentence could be using "more gentle" in the sense of "additional gentle" or in the sense of "gentle relative to X" (in this case, X might refer to the gentleness of their previous treatment or to the gentleness with which patients are treated for other conditions or to some other point of comparison). But in speech, many speakers would clarify the sense they had in mind by the stress they gave the phrase "more gentle treatment": "more gentle treatment" if the sense were "additional gentle treatment" or "more gentle treatment" if the sense were "gentle treatment relative to X."

In writing, of course, no such vocal cues are available. In their absence, some writers follow a convention of punctuation to resolve potential ambiguity: when a phrase of the form "more Y" (where Y is an adjective modifying a following noun) may be misread by some readers, they add a hyphen ("more-Y") when the intended sense is "Y relative to X" but leave the phrase open (more Y") when the intended sense is "additional Y." The Chicago Manual of Style, sixteenth edition (2010) frames the convention as follows (at 7.85):

compounds with more, most, less, least, and very [are] usually open unless ambiguity threatens.

Chicago then gives this pair of illustrative examples (involving most rather than more):

the most skilled workers (most in number)

but

the most-skilled workers (most in skill)

If every writer followed this convention, readers would never be left to wonder which sense of "more Y" the writer intended. But in my experience, a great many writers do not observe the convention—and for this reason, ambiguous instances of "more Y" are quite common. In the case of "more gentle," if the writer has shown no sign elsewhere in the book or article of abiding by the distinction between "more Y" and "more-Y," it seems to me that—in potentially ambiguous contexts—"gentler" is a better option than "more gentle" where the actual intended meaning is "gentle relative to X," and "additional gentle" is a better option than "more gentle" where the actual intended meaning is "additional gentle."