Is it really via SWO or SWD semihosting? What’s a minimal piece of code to use SWO?
Out-of-the-box, PlatformIO doesn’t have the capability to read the SWO stream (or tell OpenOCD to read it), per Viewing SWO output within PIO?. But the linked topic also contains workarounds. The “Monitior” task starts miniterm.py, which can connect to serial ports (UART) and network sockets.
Seeing semihosting output is possible by expanding the GDB initialization commands and running the application in debug mode (gd32-pio-projects/gd32-spl-semihosting-printf at main · CommunityGD32Cores/gd32-pio-projects · GitHub).