The attack's power and type change all depending on the berry that your Pokémon is holding. With a maximum power of 80, the possibilities are endless for the Pokémon. Below is the list of the Berries type and power when used in conjunction with Natural Gift. Whenever Natural Gift is used, the Berry is used as if it was used normally in battle, this gives it a one-time use possibility, unless you use moves like Recycle.
>Unburden doubles the Pokémon's Speed stat when the Pokémon's held item is lost. If there is no held item to begin with or if a held item is traded for another, Unburden will not be activated.
To clarify, using Natural Gift counts as using up the held item, in this case a berry, and thus Unburden will activate.
Hope I helped!
Source #1, Source #2