What Object are you binding too? Since you didnt provide one, i will make it up. Lets call it Users, and it has properties UserName and FullName;

Pretend you want to show the Username in the link and ToolTip the FullName, you would do this...

 <asp:GridView id="gUser" runat="server" autogeneratecolumns="False">
   <Columns>
     <TemplateField>
       <asp:HyperLink id="hlUser" runat="server" text='<%# Eval("UserName") >'  ToolTip='<%# Eval("FullName") %>'  > </asp:HyperLink>
    </TemplateField>
   </Columns>
</asp:GridView>

// in your page load event do...

   List<User>  lstUser = // some method to poplate your users

  gUsers.DataSource = lstUsers;

  gUsers.DataBind();
 

   UMMMM... WAIT WHAT?  How did i think you wanteda hyperlink. Anyways, ill leave this in there.  If you want to apply it to the template, you can do it two ways that i can think of...

   1.  You can use a panel inside the Item Template and set width and height to 100% and set tootip to that.

   2. Or you can provide a OnItemCreated Function for your GridView.

     Then in that function use e.Items  to apply atributes to the template. That will do it.