There is no package at all anymore for gd32v.
It seems PlatformIO has removed it from the registry because some of the packages it depended on were also not there.
While you can set
platform = https://github.com/sipeed/platform-gd32v.git
you will likely just run into more errors. See https://community.platformio.org/t/i-am-unable-to-install-sipeed-longan-nano-gd32v.