Solucion:
Pagina Aspx:
<asp:GridView ID="GridView1" runat="server" DataKeyNames="CarId"
AutoGenerateColumns="False" DataSourceID="SqlDataSource1">
<Columns>
<asp:BoundField DataField="CarId" HeaderText="CarId" InsertVisible="False" ReadOnly="True"
SortExpression="CarId" />
<asp:BoundField DataField="CarName" HeaderText="CarName" SortExpression="CarName" />
<asp:TemplateField HeaderText="Boton">
<ItemTemplate>
<asp:Button ID="Button1" runat="server" CommandArgument='<%# Eval("CarId") %>'
Text="Pulsar" onclick="Button1_Click" />
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
<br />
<br />
<asp:Label ID="Label1" runat="server" Text="label1"></asp:Label>
<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:SampleDataConnectionString %>"
SelectCommand="SelectCars" SelectCommandType="StoredProcedure"></asp:SqlDataSource>
<br />
y en la pagina adyacente
protected void Button1_Click(object sender, EventArgs e)
{
Button bt = (Button)sender;
Label1.Text = bt.CommandArgument.ToString();
}