use Class for HeaderStyle
<headerstyle cssclass="GridHeaderRowClass" />
.GridHeaderRowClass
{
font-family: Tahoma,Arial, Helvetica, sans-serif;
font-size: 11px;
font-weight: normal;
background-color: #303030;
color: White;
height: 25px;
padding: 0px 4px 0px 4px;
text-transform: uppercase;
height: 30px;
text-align: left;
white-space: nowrap;
}
<asp:TemplateField HeaderText="GroupName" SortExpression="GroupName" HeaderStyle-CssClass="GridHeaderRowClass "
ItemStyle-CssClass="GridCellClass TextAlignCenter" ItemStyle-Width="180px">
<ItemTemplate>
<asp:Label ID="lblGroupName" runat="server" ToolTip='<%# GetDynamicToolTip("Section",Convert.ToString(DataBinder.Eval(Container.DataItem,"GroupName"))) %>'
Text='<%# FormatColumn(Convert.ToString(DataBinder.Eval(Container.DataItem,"GroupName"))) %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>