Two quick comments: 1 - "much pleasing" is nonstandard. 2 - "pleasing" is a present participle, not a gerund.

A participle is called a "gerund" when it acts as a noun. Here's an example of "pleasing" used as a noun: "Pleasing you is my intention". In your example, "pleasing" is an adjective. We do not say "much pleasing", although I know of no purely grammatical reason not to. We just don't do it. The only adjectives we modify with "much" are comparative ones like "better" ("the song is much better"). Instead, say "the song is very pleasing" "the song is so pleasing" "the song is quite pleasing". You could say "The song has much that is pleasing to the ear". In that sentence, "much" acts as a pronoun rather than an adverb.

Yes, the AI is correct-- we would use 'very' and not 'much' with 'pleasing.' I don't know if 'much pleasing' would ever have been commonly used; it sounds like a typical mistake that some non-native speakers (or textbooks) might say. 'Very much' is commonly used, but usually in the past tense: I was very much pleased by the service at the restaurant. The help was very much appreciated. I enjoyed the meal very much.