An ACF relationship field contains an array of post IDs.
I do not know how to do this so this is a guess.
First, you would have to alter the redirect url to include the location post ID that is created in the query string for the staff post type create form and dynamically populate the location field.
Then you need to custom code an action on whatever hook gravity forms provides for when a form is submitted. In this filter you would have to update both ends of this relationship field yourself using update_field().
