Namaste
Technically speaking, the official gist or essence of BhAgvatam is the Chatuh-shloki BhAgwat , which is 4 shlokas (verses) in BhAgvat Canto 1 Chapter 2, spoken by NArAyaNa the Supreme Lord, to BramhA in the process of instructing Him about tapas (pre-meditation) to prepare for creation.
I have written about Chatuhshloki bhAgwat in this forum a few years ago - will see if I can find the link.
Basically here, NArAyaNa explains to BramhA how everything is sourced from Him, by
(i) anvay
(ii) vyatireka
which is an explanation of immanence and transcendence of NArAyaNa (VishNu) or Brahman' (clay is immanent in pot and transcends pot) , and how this will help BramhA the Creator understand the source, cause and interconnectedness, and give up the ego that "I, BramhaDev, is the Creator and created this universe". This feeling can generate 'abhimAn' and the only way to relinquish it at the position of someone like Him is to understand that NArAyaNa or Brahman' is the source of everything.
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The purpose of BhAgwat is to instill prema bhakti bhAv towards the Supreme Lord VishNu and especially His purNa Brahman avatAr of Shri KRshNa. However, BhAgwatam is deeply philosophical in explaining the oneness of existence , and it is best of both worlds because it is a perfect epitome of
bhakti
&
jnAna
he nAth nArAyaNa vAsudeva ~