Phantasy Star Adventure is waaaay too easy. Gaiden is... okay. Not really anything special. A turd is still a turd, no matter what name you give it.
Besides PS Adventure and Gaiden both being only in Japanese officially, the only reason you would want to own the GG carts of them is because you're a PS collector. You want to own PS games so badly, but once you actually buy them, you couldn't care less about them, a week later. All the Japanese ones are, are eye-candy...
Personal experience from someone who spent all of their Christmas money on importing Japanese PS stuff.