From Sega Retro
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Go Net or Go-Net (real name unconfirmed) is a rare Sega Mega Drive cartridge developed as a joint venture by Sega and Aisystem Tokyo and released exclusively in Japan in 1992. Combined with a Sega Mega Modem and subscription, it allows playing Go with other owners of the cartridge. More information in Japanese (TODO translate) appears to be on this page.
An archive of developer Aisystem Tokyo's company profile shows that a Windows version was made sometime later.