Only on specific devices, but generally every GPU released in last few generations, and every dedicated GPU from intel[1], supports "large BAR" this way to map entire GPU memory into MMIO space. This has to be supported on both target device and AFAIK the PCI Root Port[2], and of course needs to be properly handled in firmware - which this tool fixes.

[1] Intel GPUs actually can't work without it, because they do not have legacy "windowing" mode where portions of 256MB BAR window are mapped to different areas in GPU memory

[2] This tool implies that it's possible to handle root port side entirely in firmware, but it's new to me at least, so I can't vouch for that :)