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The biggest changes are to rendering. GTK 1-3 used Cairo to do CPU rendering. It is slow and incapable of modern rendering features. GTK4 makes good use of the GPU with OpenGL (and kinda Vulkan). It has been a huge win for performance improving animations, shaders, multimedia, zero-copy integration with external projects like WebKit, etc.

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Yeah, "good use", and also just happens to no longer support subpixel font antialiasing, so good luck if your screen isn't 4K. Which a lot of screens out there aren't. :-(

Was it impossible to add GPU acceleration while keeping the old API?

Yes the entire rendering API was based on Cairo.

There was an attempt to have Cairo use OpenGL but it was mostly unsuccessful and abandoned. Even if that worked the paradigms used are very different and would fight performant GPU usage.

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