Although the list of improvements/new features is small this time around, the ability to adjust the gamma setting is an excellent new addition. One scenario I could think of where this new option shines is when scanlines are applied; the user now can easily tweak the brightness of the image to his/her liking.
Chriz2600 listed the new features on his Twitter account:
DCHDMI Firmware Release v4.3
– New: RGB555 and RGB565 color expansion with optional auto detect. Difference might be subtle, really helps with slightly greenish tint, when DC is in RGB565 mode.
– New: Gamma adjust option.
For details, please refer to: https://t.co/GsDFxh80wS
— chriz2600 (@chriz2600) November 24, 2019
Make sure to check the firmware’s detailed information on the DCHMDI‘s github page: https://github.com/chriz2600/DreamcastHDMI/blob/develop/README.md