This game has been a favorite of mine since the day I first played it when I was a kid. It's an N64 title that is often overlooked when compared to its high profile competition, but thankfully it left an impression on many people who still remember it. It is an early 3D title and accomplished the format quite well, all things considered; it doesn't have quite the same masterful design as Zelda, or Mario, but it has a certain panache that makes it very endearing, and highly playable. It's also unabashedly Japanese, funny, and has a stellar soundtrack.
This game deserves to be on GOG because it's not available anywhere else. Even in its native home of Japan, Ganbare Goemon is not exactly relevant anymore. The N64 emulation is shoddy which makes it not ideal, and Nintendo never made a newer port of it. This game is way too good to be left in obscurity, and it could really use a once-over for controller support and OS compatibility. GOG is the perfect home for it.