I haven't found many threads in search about this guy, probably because Testament wasn't as big as the Big Four of Thrash, but man oh man is he a beast. Not only are Testament's albums quite well produced to let the bass be heard, but he also has a nice round bass tone that doesn't cut through so much as it slips right through. Testament might be the best example of how to produce metal to let bass and guitar have their sonic space.
On top of it all...he's got some serious chops. Fingerstyle chops at that. Not judging pickers, but it's certainly unusual in 80s thrash metal to be a plucker if your name isn't Frank Bello or Cliff Burton.
Some examples of Greg's prowess: Eerie Inhabitants Trial by Fire Practice What you Preach
And anything else off these albums.
I've heard varying things on how to get his bass tone. I know JimmyM has claimed the SVT capable of nailing it, but I'll reopen the discussion. I've seen pictures of him using Jazz Basses and pictures of him using a MM Bongo, and both are probably capable of creating that great tone. Strings are apparently DR Black Beauties.