Snorlax is located as a sleeping Pokémon in Vermillion City just outside of Diglett's Cave. The player must use a special feature of the PokéGear to wake up and battle Snorlax.
Snorlax is a bittersweet Pokémon to use in GSC. Bitter because it can only be caught very late in the game, well after facing the Elite Four, and it can be VERY tough to catch due to being at level 50 as well as the evil Rest + Snore + Leftovers combo. Sweet, because if you manage to secure the catch on Snorlax, then you can basically waltz right through the remainder of the game with very little concern for anything or anyone. Snorlax is just that good. Just get the Curse TM from this creepy guy at the top of the mansion in Celadon, teach it Rest/Sleep Talk, and then Body Slam everything into oblivion. It's literally that simple.
From this point onwards, you can fight the gyms in any order, though you will need to retrieve the Machine Parts from the Cerulean City gym before you have access to the earlier portion of Kanto. Feel free to anticipate or postpone any battles as needed.
Gym #9 - Brock (Pewter City, Rock-type): Snorlax can do okay here, but damaging Brock's Pokémon will take many Curses and time. You can take him on if you have no better options, though.
Rival (Mt. Moon, optional): Magneton is about the only one here that Snorlax may have trouble fighting, with only Normal coverage. Even then, spamming Curse still works. Everything else sans Gengar is completely powerless to Body Slam spam, and if you really want to take on Gengar too, you can temporarily teach Snorlax any special move, for example one of the elemental Punches. Else, you can teach Snorlax Earthquake.
Gym #10 - Misty (Cerulean City, Water-type): Curse and Body Slam spam. Use Amnesia and/or Rest if need be. GG!
Gym #11 - Lt. Surge (Vermilion City, Electric-type): Curse and Body Slam spam. Magneton is tougher to take down, but the boosts should be more than enough. Else, you can once again use Fire Punch or Earthquake.
Gym #12 - Erika (Celadon City, Grass-type): Again, Curse and Body Slam spam. These Pokémon have status moves, but Snorlax should have Rest, so no problem.
Gym #13 - Janine (Fuchsia City, Poison-type): If you have Earthquake, you don't even need to spam Curse here. Otherwise, maybe Curse once. Then Body Slam spam.
Gym #14 - Sabrina (Saffron City, Psychic-type): All of these Pokémon should be frail enough that Snorlax should be able to defeat them without Curses or maybe just one. If you want to play safe, using Amnesia against Mr. Mime is a good idea, as Espeon and Alakazam hit fairly hard even for Snorlax's great Special Defense.
Gym #15 - Blaine (Seafoam Islands, Fire-type): Either Earthquake or the usual Curse and Body Slam spam.
Gym #16 - Blue (Viridian City): Use the lead Pidgeot to set up Curse to the max, then sweep everything with Body Slam. Earthquake will help against Rhydon, which is otherwise likely to survive a hit. Nevertheless, after six Curses, only a critical hit can do non-negligible damage to Snorlax.
Rival (Indigo Plateau, optional): Not much has changed since the previous fight. Take on everything except possibly Gengar and Magneton. Fire Punch can help with the last two, but they can be handled better by other Pokémon too, unless Snorlax has Earthquake.
Red (Mt. Silver): Spam Amnesia, Rest, Curse, Rest, Curse, Curse... Body Slam, rinse, repeat. GG.
Snorlax already starts with most of what it needs: Rest, Snore, Body Slam and Rollout. Keep Body Slam and Rest out of these. Ignore Hyper Beam at level 57, as Snorlax doesn't need it.
TM-wise, a must have is Curse, and also Rest if you somehow had to forget that move momentarily. For Normal STAB, it can either work with Body Slam as it gets by level, or opt for the more powerful Return. The fourth and much less significant move can be Sleep Talk to help with survivability, but not entirely necessary, or Amnesia, which helps against Red. Earthquake is not entirely useful in the last few matchups, but does help Snorlax get rid of Ghost-types and Steel-types.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
How good is Snorlax in a Nuzlocke? For the vast majority of the game, Snorlax is literally nonexistent. However, if you manage to catch one, Snorlax can easily decimate all of Kanto and Red without having to grind at all, so there's that. The only possible argument against it is "wanting an ACTUAL challenge".
Neutralities: Normal, Flying, Poison, Ground, Rock, Steel, Fire, Bug, Water, Grass, Electric, Psychic, Ice, Dragon, Dark