gravityflow_assignee_choices

add_filter( 'gravityflow_assignee_choices', 'display_user_emails', 10, 2 );
function display_user_emails( $choices, $form ) {

    // Loop over "User" choices
    foreach( $choices[0]['choices'] as &$choice ) {

        // get user id
        $user_id = explode ( '|', $choice['value'] )[1] ;
        // get the user
        $user = get_user_by( 'id', $user_id );
        // update label to display email in place of the username
        $choice['label'] = $user->user_email;
    }
    return $choices;
}