how to get indexPath for cell which is located in the center of UICollectionView

Like you did yourself, indexPathForItemAtPoint is a good way of finding the index path of the element you're interested in. If your question is: how do i know the coordinates of this point? Then you should try with (using the same name you gave it in your code snippet):

initialPinchPoint = CGPointMake(self.collectionView.center.x + self.collectionView.contentOffset.x,                                 self.collectionView.center.y + self.collectionView.contentOffset.y);

This would be better code because it's cleaner and easier to read, all the content offset calculation is superfluous:

     NSIndexPath *centerCellIndexPath = [self.collectionView indexPathForItemAtPoint:[self.view convertPoint:[self.view center] toView:self.collectionView]];

This would also be the correct representation of what you're actually trying to do:
1. Taking the center point of your viewcontroller's view - aka visual center point
2. convert it to the coordinate space of the view you're interested in - which is the collection view
3. Get the indexpath that exist at the location given.

Here's what I did in Swift 3

private func findCenterIndex() {    let center = self.view.convert(self.collectionView.center, to: self.collectionView)    let index = collectionView!.indexPathForItem(at: center)    print(index ?? "index not found")}