How to Pass Data key from grid view to server side

You should bind the field into the CommandArgument of the button.
for example on the gridview, you can have a button like this one:

<asp:TemplateField HeaderText="custom column">
           <asp:Button runat="server" CommandArgument='<%# Eval("UserID") %>' OnClick="OnMyButtonClick"/>
       </asp:TemplateField>

note this button is on a TemplateField. And is using evaluation the UserID into the CommandArgument property.

Now in the C# code behind you can do this:

protected void OnMyButtonClick(object sender, EventArgs e)
   {
       Button b = sender as Button;
       if (b!=null)
       {
           var hey = b.CommandArgument;//this is my value!
           //do something now!
       }
   }

And voila..

Hope it helps.