If you are hearing gentiles with three syllables, that matches how you hear denials with three syllables. The spelling doesn’t matter, of course. What’s happening is that in many accents, the /l/ following that diphthong has an epenthetic schwa inserted right before it: [ˈd͡ʒɛntɑjəlz]. So you hear it as having three syllables:
- [d͡ʒɛn]
- [tɑ]
- [jəlz]
In other accents the diphthong can even be smoothed into a monophthong, [ˈd͡ʒɛntɐɫz], which means you now have one syllable less there without the [jə] part. You can also find accents where the /l/ is weakened into a semivowel /w/ under L-vocalization, producing [ˈd͡ʒɛntɐwz].
Accents where you sometimes hear gentiles pronounced with two syllables include Southern American English in the Deep South of the United States and in Standard Southern British in southeast England.
If you’ve ever mistaken someone saying tile for someone saying tall, then you can imagine how the two-syllable version would work in those accents. I’m guessing that your own accent probably rhymes loyal and boil — and uses two syllables for both of those. But not all accents do so, sometimes producing an apparent boil–bowl merger.
Dictionaries don’t give accurate phonetics that apply to all speakers and utterances. In many cases, they’re hopelessly out of date and misleading because they do not represent actual phonetics used by native speakers anywhere anymore.
You may also be confusing some dictionary’s hyphenation guidance as actual phonetic syllabification. Those are not the same. In writing, you are allowed to split the word gentile into gen- and -tile to break it at the end of a line. That doesn’t have anything to do with its actual syllables.
