Use eval property in your grid template as shown below.
<asp:label id="lblPriority" runat="server" text="<%# (Convert.ToInt32( Eval("Priority"))== 4 || Convert.ToInt32( Eval("Priority"))== 5 ? "No" : "Yes") %>" xmlns:asp="#unknown">
</asp:label>
full code
1. aspx
<asp:gridview runat="server" id="grid1" autogeneratecolumns="False" xmlns:asp="#unknown">
<columns>
<asp:templatefield headertext="EmployeeID">
<itemtemplate>
<asp:label id="lblID" runat="server" text="<%# Eval("EmployeeID") %> "></asp:label>
</itemtemplate>
</asp:templatefield>
<asp:templatefield headertext="Name">
<itemtemplate>
<asp:label id="lblName" runat="server" text="<%# Eval("Name") %> "></asp:label>
</itemtemplate>
</asp:templatefield>
<asp:templatefield headertext="Priority ">
<itemtemplate>
<asp:label id="lblPriority" runat="server" text="<%# (Convert.ToInt32( Eval("Priority"))== 4 || Convert.ToInt32( Eval("Priority"))== 5 ? "No" : "Yes") %>">
</asp:label>
</itemtemplate>
</asp:templatefield>
</columns>
</asp:gridview>
2.aspx.cs code
protected void Page_Load(object sender, EventArgs e)
{
var list = new List<employee>
{
new Employee() {EmployeeID = 1, Name = "Peter",Priority=2},
new Employee() {EmployeeID = 2, Name = "Jack",Priority=4},
new Employee() {EmployeeID = 3, Name = "Rich",Priority=1},
};
grid1.DataSource = list;
grid1.DataBind();
}</employee>