There is only one mention of the characters "gdb" in the 6.1.34 manual, and it's part of a debug log setting for "VBoxManage debugvm". There doesn't appear to be any mention of a GDB server.
This might be the link to the changeset?
https://www.virtualbox.org/changeset/86327/vbox
This part:
Debugger: Allow for different I/O providers instead of only TCP
So far TCP was the only option to communicate remotely with the internal debugger, the other option
was to use the console from the GUI directly. This commit reworks basic I/O to allow for different
providers where TCP is just one option. The second one being introduced is an IPC provider using a local
socket or named pipe depending on the platform. This allows for Windows kernel debugging over a pipe
using the KD stub in VirtualBox and WinDbg running on the host (not tested yet).
To my uneducated reading (never heard of a GDB anything before) this sounds like different channels for an existing server, not a new server. I'm probably wrong, though, or maybe you meant "server" to refer to these new channels.
You can check to see if the 'vboxmanage extradata' commands worked by looking in the VM's .vbox file, especially after all Virtualbox process and windows are closed. If they're present as entered, then you'll need a forum source code guru to see why they're not being picked up.