To add to what Ex-Cyber said the copyright date in anything states when those 50 or more years START and not when the copyright "expires". If it was when the copyright expires we would have a new version of everything possible (movies, books, games, software, etc) each year to update the copyright year.
As for abandonware. IT IS NOT LEGAL!!!! Once again abandonware is NOT legal. There is no such thing as legal abandonware...it's just a term invented by l33t pirates to make it seem ok to copy old games and software (kinda like that bullshit 24 hours thing ROM sites always use). Public domain is the only way that Giri Giri would become legal to distribute and mess with without Sega's (or who ever owns the copyright) permission. There is hardly any statement to suggest this has happened. And chances are that Sandi is getting a royal shitload from the Sega legal department right about now.