XmlSerialization и xsi:SchemaLocation (xsd.exe)

You'll have to do this on your own. There's no way for XML Serialization to know where you want your schema to go in any case.

Try this, though I haven't tested it yet:

[XmlRoot(ElementName = "gpx", Namespace = GPX_NAMESPACE)]
public class WhateverAGpxIs
{
    private const string GPX_NAMESPACE = "http://www.topografix.com/GPX/1/1";

    private const string XSI_NAMESPACE =
        "http://www.w3.org/2001/XMLSchema-instance";

    [XmlAttribute(AttributeName = "creator")]
    public string Creator = "ExpertGPS 1.1 - http://www.topografix.com";

    [XmlNamespaceDeclarations]
    public XmlSerializerNamespaces Namespaces =
        new XmlSerializerNamespaces(
            new[]
                {
                    new XmlQualifiedName("xsi", XSI_NAMESPACE),
                    new XmlQualifiedName(string.Empty, GPX_NAMESPACE)
                });

    [XmlAttribute(AttributeName = "schemaLocation",
        Namespace = XSI_NAMESPACE)]
    public string SchemaLocation = GPX_NAMESPACE + " " +
                                   "http://www.topografix.com/GPX/1/1/gpx.xsd";

    [XmlAttribute(AttributeName = "version")]
    public string Version = "1.1";
}