Hi manionasp,
Please refer below sample.
HTML
<asp:GridView ID="gvCustomers" runat="server" AutoGenerateColumns="false" AllowPaging="true" OnPageIndexChanging="gvCustomers_OnPageIndexChanging"
    PageSize="5">
    <Columns>
        <asp:BoundField DataField="ContactName" HeaderText="ContactName" />
        <asp:BoundField DataField="City" HeaderText="City" />
        <asp:BoundField DataField="Country" HeaderText="Country" />
    </Columns>
</asp:GridView>
<asp:Button ID="btnGet" Text="Get" runat="server" OnClick="GetPageNumber" />
<asp:Label ID="lblPgeNumber" runat="server" ForeColor="Red" />
Namespaces
C#
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
Code
C#
protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.BindData();
    }
}
private void BindData()
{
    using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["conString"].ConnectionString))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT ContactName, City, Country FROM Customers", con))
        {
            using (SqlDataAdapter da = new SqlDataAdapter(cmd))
            {
                DataTable dt = new DataTable();
                da.Fill(dt);
                this.gvCustomers.DataSource = dt;
                this.gvCustomers.DataBind();
            }
        }
    }
}
protected void gvCustomers_OnPageIndexChanging(object sender, GridViewPageEventArgs e)
{
    this.gvCustomers.PageIndex = e.NewPageIndex;
    this.BindData();
}
protected void GetPageNumber(object sender, EventArgs e)
{
    int pageNumber = gvCustomers.PageIndex;
    lblPgeNumber.Text = "Current Page Number is : " + pageNumber.ToString();
}
Screenshot
