The Gen 7 buff is a stat change affecting certain Pokémon from Generations I through V and increasing their stats significantly in order to make them more competitive. The change occurred with the release of Pokémon Sun and Moon. You can see the whole list of updated stats on Serebii.
For the second time since Gold & Silver introduced the Special Defense stat, multiple Pokémon have had changes to their initial base stats. These changes are relatively minor, just focusing on a slight increase in one stat. However, these changes do change the way the Pokémon is to be utilised in battle.
In Pokémon GO, this buff will be reflected as a CP adjustment. Currently, Pokémon GO is using Generation 6 stats, not Generation 7 stats, so this buff is not yet active.
Here’s how stats and Max CP will look like after the Gen 7 buff is applied:
|Pokemon||Gen||Attack||Defense||Stamina||Old CP||New CP (Gen 7)|