What alternative are there outside of the outdated win32 gdi api?

You can write your own, using doing some research into existing algorithms.

I'd imagine (but don't know for sure), that most of those Blt functions are designed for speed - doing the resize every frame.

If you're converting a 400x400 to 100x100, you just need to do that once and then draw the new 100x100 image using the regular Blt functions. You can do this on the CPU, using a normal function of your own writing, and it should be plenty fast enough.

Some algorithms are designed for upscaling, others may be better suited for downscaling.

For fast blits, the algorithm may be just using percentages to choose what one pixel to sample (i.e. 0.3 horizontally is the color 'red', so draw red!).

For better resizes, other algorithms take into account multiple nearby pixels and combine the colors to generate a superior image. This is probably what you want.

More "intelligent" algorithms try to analyze the image to find "edges" and other formations of pixels that stand out, and then aim to recreate these formations.