There are very few scenarios where I'd ever take Gengar over Golem, and Snorlax is not one of them. There's very little upside to going Gengar -> Egg, because it's a stupid easy read and it's laughable when people try it. There is very big downside though, both on the initial switch, and the subsequent switch. Gengar's problem is that it's not a threat initially, so what if Snorlax DOESN'T EQ? What's "forcing" it to EQ? You draining? You tossing? A psychic maybe?
Gengar's good for Persian, that's about it. Golem has it beat elsewhere.
Chansey doesn't wall Lapras as easily as it does Starmie. I'm not a big fan of Sing Lapras, so that leaves 2 options for the 4th move: Confuse Ray, and Hyper Beam. Chansey is in KO range for BS/HB from 65% on the switch, which is not terribly unlikely by any standard. An FP brings that up to 85-90%. And Cray luck is very viable; 37% chance to move per turn isn't great. And Chansey is probably cutting its losses pretty early, not wanting to gamble <40% if it wants to do other things. Cray can definitely force Chansey out on the reg. Chansey is not walling Lapras all day.
An active lax is probably the single most dangerous poke from the get-go (unless you have the odd aero, slowbro, cloy, etc). Unlike Tauros, it makes very little sense to sacrifice something just to para it (although I'd gladly take a BS with Chansey to para the thing). And despite missing Blizzard, it's coverage barely suffers. And again, doing that little chain switch with Gengar -> Egg is not reliable because of the risk involved. There's just as much reason for lax to EQ your active Chansey as much as there is Body Slam; maybe it's checking for Counter. Furthermore, if you DO get Gengar in pain-free, what now? There's not a single move in Gengar's arsenal that threatens snorlax to the point where it feels like it needs to EQ.
Lax is almost ALWAYS at least 2 for 1 with SD considered. And moreover, lax has plenty of tricks up its sleeve. Amnesia, Counter, Surf, Reflect, Rest, etc makes it all the merrier.
Snorlax is a different kind of good in RBY than in GSC. All the paras/sleep floating around usually makes Snorlax outspeed 3-4 of the opposition with regularity, something you never see in GSC.
Reflect zam is great... but only against the physical teams. You're better off with another 4th move against the "old" RBY teams, who have no plans of breaking through the physical side outside of Tauros anyway.
But Golem's better than Gengar so none of that's not really important.