For all who are interested I have set up a small VexRiscV System with Wishbone Bus running on a BASYS3-Board from Digilent. The specific feature of this system is the embedded GDB-Server in Hardware which receives the RSP-commands from the GDB and creates suitable commands for the VexRiscV debug bus. Thus the GDB can directly connect to the system via serial port without a software GDB-Server (like e.g. OpenOCD or others). The Project is available via Github: https://github.com/BLangOS/VexRiscV_with_HW-GDB_Server. It is no problem to port the System to other platforms. Have fun.
Interesting approach to put this into pure hardware, but I strongly believe you'll get more 'love' with implementing a proper JTAG protocol directly to the core (avoiding the puzzling RISC-V debug interface design). I tend to stay away from the serial gdb server protocol as far as I can due to stability issues seen in the past. When it comes to debugging: Keep it as simple as possible, use in-circuit-emulation only.
From my viewpoint the "the puzzling RISC-V debug interface" is a very interesting interface directly into the core of the VexRiscV, which allows to adapt different frontends for debugging. If you don't want to spend much hardware for the debug interface, you are definitely better off with JTAG. There is already a JTAG interface available for the VexRiscV processor and a related OpenOCD port. The approach presented here serves to make access to the processor from the development environment view "as simple as possible". Beginners in particular often find it very difficult to put the necessary environment of debugger, GDB server and suitable debug interface together to debug self-designed processor systems in FPGAs. The presented GDB server in hardware simplifies experimenting with the very interesting VexRiscV. With the Basys3 board, you only need a single USB connection to the PC. The board is first configured via this connection and then the GDB debugger can be directely connected to the GDB server via the COM port of the same USB connection. I am surprised that you address stability issues of other systems you have seen in the past at this point. This GDB-Server runs very stable. This has to be the case, since it is also used in training courses for beginners in FPGA design and computer organization.
Great project! However, I also prefer JTAG ;) In case you are interested, here is a RISC-V compliant debug transport module (JTAG TAP): https://github.com/stnolting/riscv-debug-dtm This module has been successfully integrated (together with a compatible "debug module") into an open RISC-V soft-core SoC: https://github.com/stnolting/neorv32
Thanks for this very interesting hint.