Just caught a GDI+ error running System.Drawing on a Mac? Maybe I got what you need here, to solve it in 5 secs.
If you are trying to generate thumbnails on .NETCore, you probably have come across of the porting of the System.Drawing namespace into mono.
If you haven't, check it out. It's a fantastic article about the several options for image processing on .NET Core that has some limitations.
I got the whole thing up and running in debug. But when I went for our dotnet run, I got the following crash:
The type initializer for ‘System.Drawing.GDIPlus’ threw an exception. — -> System.DllNotFoundException: Unable to load DLL ‘gdiplus’: The specified module or one of its dependencies could not be found.
After googling a bit, I found out that macs have a brew package for the GDI+ good to go.
brew install mono-libgdiplus
And it works like a charm!
Hope this tiphelps you too, and saves you some minutes.