Use Conditions with Eval in ItemTemplate of ASP.Net GridView

Ref: Using IF ELSE Condition with EVAL function in ASP.Net GridView using C# and VB.Net

You can do as following

<asp:GridView ID="GridView1" runat="server">
        <Columns>
            <asp:TemplateField>
                <ItemTemplate>
                    <asp:Label ID="Label1" runat="server" Text='<%# Eval("Data1") != null ? Eval("Data1").ToString() : Eval("Data2").ToString()%>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>