Guild Wars 2

What do you get if you use it on a level 80 character? Are there rewards of gear that come with it?

Here is the Anet L80 boost tutorial video and this issue is discussed specifically at just about the 5 minute mark in the video.

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And the support page discussing this.

https://help.guildwars2.com/entries/99059447-Level-80-Boost

What if all my characters are already level 80?

You can choose to either create a new character or to consume the boost with one of your existing level 80 characters. Using the boost on a level 80 character will deliver the package of rewards that is normally delivered when making the boost permanent. However, there is no trial for an existing level 80 character, so the item will be permanently consumed if it is used by a level 80 character. You will need to confirm twice before the item is consumed if you want to use it on an existing level 80

At the very least you get the shared inventory slot it’s sitting in.