GpxSerialization.cs

using System;
using System.Xml;
using System.Xml.Serialization;

namespace NetTopologySuite.IO
{
    public static class GpxSerialization
    {
        private static readonly XmlSerializer GpxSerializer = new XmlSerializer(typeof(gpxType));

        public static gpxType ReadGpx(XmlReader reader) =>
            GpxSerializer.Deserialize(reader ?? throw new ArgumentNullException(nameof(reader))) as gpxType;

        public static void WriteGpx(XmlWriter writer, gpxType gpx) =>
            GpxSerializer.Serialize(writer ?? throw new ArgumentNullException(nameof(writer)),
                                    gpx ?? throw new ArgumentNullException(nameof(gpx)));

        /// <summary>
        /// Recomputs the bounds of this <see cref="gpxType"/> according to its point data.
        /// </summary>
        /// <param name="gpx">This <see cref="gpxType"/> instance whose bounds to recompute.</param>
        /// <returns><paramref name="gpx"/>, with its <see cref="metadataType.bounds"/> recomputed according to its point data.</returns>
        public static gpxType RecomputeBounds(this gpxType gpx)
        {
            var bounds = new boundsType
            {
                minlat = Decimal.MaxValue,
                minlon = Decimal.MaxValue,
                maxlat = Decimal.MinValue,
                maxlon = Decimal.MinValue,
            };

            UpdateBounds(bounds, (gpx ?? throw new ArgumentNullException(nameof(gpx))).wpt);

            if (gpx.rte != null)
            {
                foreach (var rte in gpx.rte)
                {
                    UpdateBounds(bounds, rte.rtept);
                }
            }

            if (gpx.trk != null)
            {
                foreach (var trk in gpx.trk)
                {
                    if (trk.trkseg != null)
                    {
                        foreach (var trkseg in trk.trkseg)
                        {
                            UpdateBounds(bounds, trkseg.trkpt);
                        }
                    }
                }
            }

            if (gpx.metadata == null)
            {
                gpx.metadata = new metadataType();
            }

            if (gpx.metadata.bounds == null)
            {
                gpx.metadata.bounds = bounds;
            }
            else
            {
                gpx.metadata.bounds.minlon = bounds.minlon;
                gpx.metadata.bounds.maxlon = bounds.maxlon;
                gpx.metadata.bounds.minlat = bounds.minlat;
                gpx.metadata.bounds.maxlat = bounds.maxlat;
            }

            return gpx;
        }

        // wptType[] instead of IEnumerable<wptType> because it cuts down on
        // virtual calls and allocations, the JIT emits faster code, and
        // we can get away with it because this is private.
        private static void UpdateBounds(boundsType bounds, wptType[] points)
        {
            if (points == null)
            {
                return;
            }

            foreach (var point in points)
            {
                if (point == null)
                {
                    continue;
                }

                if (bounds.minlon > point.lon)
                {
                    bounds.minlon = point.lon;
                }

                if (bounds.minlat > point.lat)
                {
                    bounds.minlat = point.lat;
                }

                if (bounds.maxlon < point.lon)
                {
                    bounds.maxlon = point.lon;
                }

                if (bounds.maxlat < point.lat)
                {
                    bounds.maxlat = point.lat;
                }
            }
        }
    }
}

NetTopologySuite.IO.GPX.csproj

<?xml version="1.0" encoding="utf-8"?>
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <SolutionDir Condition=" '$(SolutionDir)' == '' ">$(ProjectDir)..\..\</SolutionDir>
    <TargetFrameworks>net35-client;net40-client;net403-client;net45;netstandard1.3</TargetFrameworks>
    <RootNamespace>NetTopologySuite.IO</RootNamespace>
    <SignAssembly>true</SignAssembly>
    <AssemblyOriginatorKeyFile>$(SolutionDir)nts.snk</AssemblyOriginatorKeyFile>
  </PropertyGroup>

  <Import Project="$(SolutionDir)NetTopologySuite.Common.props" />

  <ItemGroup>
    <Compile Include="$(SolutionDir)SharedAssemblyVersion.cs" Link="Properties\SharedAssemblyVersion.cs" />
  </ItemGroup>

  <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard1.3' ">
    <PackageReference Include="System.ComponentModel.Primitives" Version="4.3.0" />
    <PackageReference Include="System.Diagnostics.Debug" Version="4.3.0" />
    <PackageReference Include="System.Diagnostics.Tools" Version="4.3.0" />
    <PackageReference Include="System.Runtime.Serialization.Formatters" Version="4.3.0" />
    <PackageReference Include="System.Xml.XmlDocument" Version="4.3.0" />
    <PackageReference Include="System.Xml.XmlSerializer" Version="4.3.0" />
  </ItemGroup>
</Project>