Why can't I select 3840 × 2160p30 (4K30) in the software?

This is a hardware limitation of the GC750.
When the input signal frame rate is higher than 60 FPS, high-quality upscaling is not supported.
For example, a 1080p120 input signal cannot be upscaled to 4K30 (3840 × 2160p30).
We recommend setting the source output resolution and frame rate to match the capture resolution and frame rate selected in the software.