Filter Grid view en Model Search

public 

add to safe in rules

 public function rules() {
    return [
        ....
        [['locatie'], 'safe'],
    ];
}

add in join

// locatie is attribute from student
$query = Gesprek::find()
            ->joinwith(['student'])

note that student is relation

public function getStudent()
{
   return $this->hasOne(Student::className(), ['id' => 'studentid']);
}

add in andFilterWhere

In Search Model for sort

add sort attributes

In view

add column