When you reach the champion in Pokemon Gold, you have to defeat his Pokemon. His 4th one, Aerodactyl, knows the move Rock Slide. Aerodactyl cannot learn this move, and it does not appear as a TM in this generation.
There are some secret trainers near Goldenrod City. To get to them, surf on the water next to the Daycare House. Keep going down until you reach land. These trainers have nothing special, they just train your Pokemon and give you money. However, the trainer facing up will give you Soft Sand after defeating her, which ups the power of ground moves. There is also some rare candy on the island a bit further up. It is in a tree.