It seems I didn't understand you properly. Do you mean centralize only header column cell but not others, right?
I would suggest you to use getHeaderCell method of GridView in the its AfterRender event listener.
Example
<%@ Page Language="C#" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<script runat="server">
protected void Page_Load(object sender, EventArgs e)
{
if (!X.IsAjaxRequest)
{
Store store = this.GridPanel1.Store.Primary;
store.DataSource = new object[] {
new object[] {"test1", "test1", "test1"},
new object[] {"test2", "test2", "test2"},
new object[] {"test3", "test3", "test3"}
};
store.DataBind();
}
}
</script>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>Ext.Net Example</title>
</head>
<body>
<form runat="server">
<ext:ResourceManager runat="server" />
<ext:GridPanel ID="GridPanel1" runat="server" AutoHeight="true">
<Store>
<ext:Store runat="server">
<Reader>
<ext:ArrayReader>
<Fields>
<ext:RecordField Name="test1" />
<ext:RecordField Name="test2" />
<ext:RecordField Name="test3" />
</Fields>
</ext:ArrayReader>
</Reader>
</ext:Store>
</Store>
<ColumnModel runat="server">
<Columns>
<ext:Column DataIndex="test1" Header="Test1"/>
<ext:Column DataIndex="test2" Header="Test2"/>
<ext:Column DataIndex="test3" Header="Test3"/>
</Columns>
</ColumnModel>
<View>
<ext:GridView>
<Listeners>
<AfterRender Handler="this.getHeaderCell(1).align = 'center'"/>
</Listeners>
</ext:GridView>
</View>
</ext:GridPanel>
</form>
</body>
</html>
