See How to recompile framework or update it? - #2 by maxgerhardt. PlatformIO operates on a CDN so there’s no single server location.
The lateset version is e.g. https://dl.registry.platformio.org/download/platformio/tool/toolchain-xtensa32/2.80400.210211/toolchain-xtensa32-windows_amd64-2.80400.210211.tar.gz.
There is no listed GCC 9 option. 8.4.0 is currently the latest.
Using platform_packages you can redirect the package source of toolchain-xtensa32 to a location on disk or an online repository. All you need is the compiled compiler package with a package.json sticked into the root of that directory with the version info. That file can e.g. be derived from the current version as I’ve linked above (or look in <home>/.platformio/packages/toolchain-xtensa32/package.json).