You have to trade it to an other game for it to evovle.
Haunter can only be evolved into Gengar via Trading (such is the case in all Pokemon titles that include Haunter, not just Silver).
Haunter evolves into gengar once you trade him.
When you trade him/her like in all other generations.
You don't need to get a gastly to evolve because you can catch haunters in Pokemon tower
There is no NPC who you can trade a Haunter to for a Gengar in Pokemon Crystal. To get Gengar you will need to trade with another player or a copy of Pokemon Gold, Silver or Crystal.
Gastly evolves into Haunter at Lv25.
Haunter is one of a few Pokemon that needs to be traded to evolve. A list of a few others Graveler, Machoke, Kadabra. Those are just off the top of my head. But you will need to trade it to a friend and have them trade it back.
Machoke, Kadabra, Graveler, Haunter, Poliwhirl, Scyther, Onix, Slowpoke, Seadra, Porygon.
you have to trade it back and forth with another trainer to evolve it
Kabuto doesn't evolve with a stone. It will evolve into kabutops at level 40.
I'm not sure. But if you catch a Gastly in either Burned Tower (Ecruteak City) or in Sprout Tower (Violet City) and train it, you Gastly will evolve into Haunter at level 25. To get Gengar, trade your Haunter and it becomes Gengar.
haunter does