Don't truncate the string; use CSS to hide the overflow instead.
ASPX file:
<asp:TemplateField ItemStyle-CssClass="hide-overflow">
<ItemTemplate>
<asp:Label runat="server"
Text='<% Eval("DataItem.SKILLS") %>'
ToolTip='<% Eval("DataItem.SKILLS") %>'
/>
</ItemTemplate>
</asp:TemplateField>
Stylesheet:
.hide-overflow
{
overflow: hidden;
text-overflow: ellipsis;
white-space: nowrap;
}