Why datapager with gridview
DataListView already supports datpager in asp.net 3.5 and everything you want you can get from the datalistview. Still why to use datapager with gridview. Gridview doesnot supports datapager bydefault. You have to extend gridview implementing IPageableContainer.
If you are migrating from old asp.net 2.0 to asp.net 3.5 and you have used gridview in [...]

One of the new controls that shipped with ASP.NET 3.5 is the DataPager. The big advantage with the DataPager is that it is decoupled from the data-bound control it provides paging support for. It can be placed anywhere within the data-bound control’s control hierarchy (like contained within the LayoutTemplate of the ListView). It [...]

Gridview is one of the advanced and mostly used control on asp.net. Bellow you can find some of the interesting features of gridview.
Paging  support:
Gridview control has built in paging and sorting support that provides developer less time to configure paging and sorting to present data on tabular format. Paging and sorting can be configured in [...]