What font renderer do you mean? I don't know about the internals of Microsoft renderers but vector graphics and font rasterization generally are distinct disciplines. This has started to change with higher-resolution displays, but font rasterization traditionally has been a black art involving things like grid snapping, stem darkening etc. Probably (but can't back this up) the most used font rasterization technologies are still Microsoft ClearType (are there implementations that use GPU??) and Freetype (strictly a CPU rasterizer). Don't know about MacOS, but I heard they don't do any of the advanced stuff and have less sharp fonts on low-dpi displays.
I would also like to know where Loop-Blinn is used in practice? I once did an implementation of quadratic Beziers using it, but I'm not up to doing the cubic version, it's very complex.
Its a blackbox. But Microsoft is very clear that its "hardware accelerated", whatever that means (IE: I think it means they got GPU-shaders handling a lot of details).
GDI / etc. etc. are legacy. You were supposed to start migrating towards Direct2D and DirectWrite decades ago. Cleartype itself moved to DirectWrite (though it still has GDI renderer for legacy purposes).
I would also like to know where Loop-Blinn is used in practice? I once did an implementation of quadratic Beziers using it, but I'm not up to doing the cubic version, it's very complex.