How to get the index path of a UICollectionView header?

You should make your own dictionary mapping index paths to header views. In your collectionView:viewForSupplementaryElementOfKind:atIndexPath: method, put the view into the dictionary before returning it. In your collectionView:didEndDisplayingSupplementaryView:forElementOfKind:atIndexPath:, remove the view from the dictionary.

override func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView {
    switch kind {
    case UICollectionElementKindSectionHeader:
        let headerView = collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: "header", for: indexPath) as! HeaderCollectionReusableView
        let gestureRecognizer: UITapGestureRecognizer = UITapGestureRecognizer(target: self, action: #selector(didSelectSection(gesture:)))
        headerView.addGestureRecognizer(gestureRecognizer)
        return headerView
    }
}

Now in didSelectSection :

let indexPaths = self.collectionView?.indexPathsForVisibleSupplementaryElements(ofKind: UICollectionElementKindSectionHeader)
for indexPath in indexPaths! {
    if (gesture.view as! HeaderCollectionReusableView) == collectionView?.supplementaryView(forElementKind: UICollectionElementKindSectionHeader, at: indexPath){
        print("found at : \(indexPath)")
        break
    }
}