Unlike situations such as Nintendo not including the names of the original 2002 devs in the credits for Metroid Prime Remastered (who get a blanket credit of "based on the work of the original Metroid Prime development team"), this situation appears to be a case of Sega purposely removing credits out of spite. It's been noted that the most prominent names to be excluded from the credits of the GOG releases are developers who left Sega a couple years ago to form a new independent studio funded by NetEase, most notably Yakuza/Like a Dragon series creator and longtime creative director Toshihiro Nagoshi.
Like I said, while the Nintendo and Metroid Prime Remaster feels like a simple situation of "only wanting to directly credit who worked on the remaster while the original devs get a simple blanket credit that the remaster is based on their work," THIS situation feels like Sega unfortunately pulling a bit of a Konami and removing credits out of spite for these devs leaving the company.