Hi,
The problem with raw telnet is that it does not support easy file transfer and has no convenient API for running just one command and seeing when it exited. It could be achieved via various workarounds, but they won’t be very reliable.
We currently recommend setting up a custom project and simply adding custom pre-debug commands to transfer your files to the device. Similarly, you can use the expect tool to connect to the target via raw telnet and specify it as the local GDB executable so that VisualGDB will simply launch it and send the commands there.
We will monitor the demand for this feature and consider adding it if the demand grows. In the meanwhile, feel free to try the workaround described above and if it does not work, we will gladly provide more detailed steps.