The way you are working class is wrong.
Do like this.
protected void btn2(object sender, EventArgs e)
{
List<percentage> objPercentage = new List<percentage>();
objPercentage.Add(new Percentage{ id="098", DateTime="2014/02/0", Percent="78"});
objPercentage.Add(new Percentage{ id="099", DateTime="2014/02/0", Percent="68"});
objPercentage.Add(new Percentage{ id="099", DateTime="2014/02/0", Percent="68"});
GridView1.DataSource = objPercentage;
GridView1.DataBind();
}