You can access the control in Button click event
protected void GetDetails(object sender, EventArgs e)
{
GridViewRow row = (sender as Button).NamingContainer as GridViewRow;
string id = "Id: " + row.Cells[0].Text;
string name = "Name: " + row.Cells[1].Text;
string country = "Country: " + (row.FindControl("ucCountry") as Country).Country;
string message = string.Format("{0}\\n{1}\\n{2}", id, name, country);
ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('" + message + "');", true);
}