Ground-type Pokémon

Pure Ground-types

Immune to Electric attacks; strong against Poison and Rock attacks; weak against Grass, Ice and Water attacks

Sandshrew
Sandslash
Diglett
Dugtrio
Cubone
Marowak
Phanpy
Donphan

Ground/Flying-types

Immune to Electric and Ground attacks; strong against Bug, Fighting and Poison attacks; weak against Ice and Water attacks

Gligar

Ice/Ground-types

Immune to Electric attacks; strong against Poison attacks; weak against Fighting, Fire, Grass, Steel and Water attacks

Swinub
Piloswine

Ground/Poison-types

Immune to Electric attacks; strong against Bug, Fighting, Poison and Rock attacks; weak against Ground, Ice, Psychic and Water attacks

Weak against Bug attacks in Red, Yellow and Blue Versions

Nidoqueen
Nidoking

Rock/Ground-types

Immune to Electric attacks; strong against Fire, Flying, Normal, Poison and Rock attacks; weak against Fighting, Grass, Ground, Ice, Steel and Water attacks

Geodude
Graveler
Golem
Onix
Rhyhorn
Rhydon
Note: Rhyhorn and Rhydon are actually Ground/Rock, not Rock/Ground
(this has no effect on game play; not even GS's type search function cares)
Larvitar
Pupitar

Steel/Ground-types

Immune to Electric and Poison attacks; strong against Bug, Dark, Dragon, Flying, Ghost, Normal, Psychic, Rock and Steel attacks; weak against Fighting, Fire, Ground and Water attacks

Steelix

Water/Ground-types

Immune to Electric attacks; strong against Fire, Poison, Rock and Steel attacks; weak against Grass attacks

Wooper
Quagsire