How do you obtain the eighth gym badge in Pokemon Diamond and Pearl?
By beating the gym leader
there are 150 kinds of Pokemon that you can see and/or obtain. Unless you have friend trade you Pokemon that you can only get in pearl to you, you'll never obtain them all. In Pokemon diamond and pearl all you have to do is see the Pokemon and you get its data unlike the older versions where you have to obtain them.
There are only three ways to obtain Groundon in Pokemon Diamond, Pearl and Platinum and that is to either Migrate it from Pokemon Ruby, Sapphire (only obtainable on Sapphire if it has been traded from Ruby or Emerald) or Emerald, obtain it through a trade or hack it, which is not advised but a solution.