Hi,
There is an open sourced implementation of GDI+ for Linux, Windows, MAC, as listed here on Github.
C-based implementation of the GDI+ API. Contribute to mono/libgdiplus development by creating an account on GitHub.
github.com
So it's not strictly limited to being a Windows API, although it may have started off life as a Windows API, this is a Mono and it's a cross platform, open source .NET framework, as stated on their website, so can easily be implemented by the OBS dev team for GDI Text in the Linux and MAC version of OBS.