Before playing this, is this part of the witcher timeline in any way or is this a separate story?

A little late maybe but just finished the game and this story happens before the witcher games timeline, as far as I can tell.

It's a separate story but you get to meet some characters from the witcher games ;)

It's a seperate story. It takes place during II Northern War, so in the same time as books (there are even some scenes from the book in the game) and before the games