In your pageindexchanging event you wrote following lines
GridPrevistos.PageIndex = e.NewPageIndex
GridPrevistos.DataBind()
But when writing GridPreVistos.DataBind(), you need to specify the data source again (as you may have provided when you bound this grid for the first time).
in simple words, after you specified the current pageIndex for the gridview you need to bind it again with dataSource. This time it will show the new page as value in e.NewPageIndex.
Hope this will help you.