Because of great overall stats, especially in HP, combined with his powerful moveset, Snorlax is one of the most common Pokemon you will see defending Gyms.
Tips and strategies
Effective attack types
Snorlax's only weakness are Fighting-type attacks, making him quite annoying to deal with, since he has a lot of HP and there are not that many good Fighting-type Pokemon. Luckily, he is also only resistant against Ghost-type attacks, which are also not that common. Knowing this, it might be better to bring a strong Pokemon to do neutral damage against him, than to bring a weak Fighting-type Pokemon that will do super effective damage against him.
Snorlax has a wide variety of attacks so choosing a Pokemon that would resist every attack in Snorlax's moveset might prove to be difficult. Snorlax usually has Zen headbutt as his normal attack, making th use of Fighting-type Pokemon not a very good choice. Then there's Earthquake, which is not only super effective against many Pokemon, it also deals huge damage with a neutral type effectiveness.