SendPacketToAdapter can send any packet on the network even if it is not in correct format. I think you just don’t form the correct INTERMEDIATE_BUFFER. I’m not sure I fully understand what you are doing in your C# code as I’m not really experienced in C#, but to my limited knowledge the code is incorrect.
byte newPacket = new byte[sizeof(ETHER_HEADER) + sizeof(IPHeader) + sizeof(UdpHeader) +
sizeof(PlatformHeader) + dataLength + sizeof(GPSHeader)];
IntPtr outgoingPacket = Marshal.AllocHGlobal((IntPtr)(newPacket.Length));
Marshal.Copy(newPacket, 0, outgoingPacket, newPacket.Length);
ETH_REQUEST outgoingRequest = new ETH_REQUEST();
outgoingRequest.hAdapterHandle = Request.hAdapterHandle;
outgoingRequest.EthPacket.Buffer = outgoingPacket;
In your code above you use newPacket as managed INTERMEDIATE_BUFFER and outgoingPacket as unmanaged INTERMEDIATE_BUFFER. However, the size for allocation for newPacket must be sizeof(INTERMEDIATE_BUFFER), not the size you have used above. As well when you copy managed INTERMEDIATE_BUFFER into unmanaged memory once again you should copy sizeof(INTERMEDIATE_BUFFER).
Note that you have to initialize INTERMEDIATE_BUFFER fields (you can copy values from the original structure and adjust m_Length).
Also note that the resulted packet must fit into MAX_ETHER_FRAME (more exactly it should not exceed network interface MTU value).