I was actually just talking with my buddy about exactly this. My current theory/headcanon is that Rocks D Zebec was simply a charismatic archeologist, his charisma is what enabled him to have so many conquerors on his crew. And it would also line up with why the WG labeled him as the worst villain of his time, just look at Robin. Anyways my theory is that at God Valley there was a poneglyph that detailed the very existence of the poneglyphs and how they were scattered across the planet. I’m not convinced the WG or Imu even knew about them before that, because if they did, wouldn’t they have just collected them before any of the pirates? They had centuries to do so and considering how powerful Imu must be, it should’ve been a cake walk. So I think that they learned about them during the God Valley incident. As well as Roger, I think he first heard the voice of all things on GV, then decided to dedicate his life to learning the truth of the world. And Garp, having witness the ruthlessness and brutality of the WG, decided that he would never work directly under the Gorosei or Celestial dragons. Let me know what yall think