First off, you cannot befriend the Crab People until you learn the Nagasaki fart while in Canada. For the Gnomes, you will have to wait until you complete the storyline quest "Defeat The Underpants Gnomes".
How to befriend The Gnomes- After completing the "Defeat The Underpants Gnomes" quest, there will be a Underpants Gnome in your room. He will give you a quest where you will have to find 5 underpants. These are the locations of them-
-Underpants in Stan's room in a drawer
-Underpants in Stan's parents bedroom in a drawer
-Underpants in Cartman's house in his mother's bedroom in a drawer
-Underpants in Kyle's room in a drawer
-Underpants in Kevin Stoley's bedroom in a drawer. Kevin Stoley's house is the dark blue house that has a fast travel flag infront of it. It is two houses down to the right of the South Park Community Center.
How to befriend the Crab People- Like i've said, you have to learn the Nagasaki fart first from Terrence & Phillip in Canada. Next, go to the sewers which is located to the left of the Post Office. While in the sewers, go through the entire section and keep going down all the ladders there. Eventually you'll come to a cave section and you'll hear the Crab People singing. Go to the right in the caves and you'll see a Crab Person playing on a drum. Use a Nagasaki fart on the rock next to the Crab Person and talk to him and he will friend you.