Gengar - the very first Ghost Pokemon, at a time when the Ghost-type was mostly a joke, aside from their immunities to Normal and Fighting. After all, they were meant to be super-effective against the broken Psychics, but not only did a bug mean their moves were ineffective instead (to say nothing of the complete lack of actual Ghost attacks), but also because of their Poison-type, they were weak to the very Psychics they were facing! And yet, despite all this, Gengar still proved to be a strong Pokemon, due to high special offensive stats and a wide movepool. Future generations would slowly but surely add to its power; the Ghost-type being physical and Gengar having low Attack meant Shadow Ball would at best be a consolation move on rare physical builds, but Gengar's large movepool gave him plenty of choices otherwise, anyways. In 4th-generation, Shadow Ball became special-type, and Gengar procured usable STAB. His movepool remained massive, and he is now a potent offensive threat; add in three immunities to common types that give him many switch-in opportunities and Gengar is all set to take on the world.
Capture/Training
The first and easiest place to capture Gastly in HG/SS is Sprout Tower.
In training, Gastly evolves into Haunter at Level 25. Delaying said evolution 4 levels to learn Shadow Ball 4 levels earlier is suggested, as Gastly has a bit of a hard time doing damage otherwise. Moves past Shadow Ball in the level-up set aren't much to speak of. After that, it's just a matter of doing the trade that brings about Gengar.
Stats
60 HP, 65 Atk, 60 Def, 130 SpA, 75 SpD, 110 Spe
Dangerously powerful Special Attack and high Speed makes Gengar a prime candidate for a special sweeper. Defenses are low, though, so don't let him get hit a lot.
Abilities
Levitate is already one of the best abilities in the game, but Gengar especially enjoys the ability: it removes one of its weaknesses (and to a common type, at that) and even makes it an immunity, giving Gengar an easy switch-in opportunity right there. Add on its Normal and Fighting immunities and Gengar will easily find switch-in opportunities despite its poor defenses.
Moves
First things first: Shadow Ball. Its power is destructive, and its coverage is solid. Technically, Gengar's other (newly-Special) STAB move, Sludge Bomb is even more powerful, but it really lacks coverage.
Gengar has a great secondary attack movepool. Of those attacks, probably his most important is Focus Blast. Horrible accuracy notwithstanding, it combines well with Shadow Ball to dish out at least neutral damage to everything; it also nicely deals with the Dark and Steel types that commonly give it trouble. Of the remaining moves, Thunderbolt is always good (while Thunder is good in the rain), and Energy Ball is decent too. Hidden Power is always an option if you're lucky enough to get a good type (Fire is probably best). Icy Wind has certain niche uses.
While Gengar isn't much of a physical attacker with only base 65 Attack, he nevertheless has a couple of physical attacks to use. Focus Punch hits certain troublesome specially-defensive Normal-types hard. And, of course, there's always Explosion.
Support moves for Gengar are as follows. First, Hypnosis. Even with the accuracy drop back to 60%, it's still a good move for potentially shutting down an opponent. Will-o-Wisp is also an option, to help reduce the impact of physical moves on Gengar.
Substitute is always an option, but particularly in the metagame, where it acts as a useful buffer for prediction against certain Pursuit-using threats.
Pain Split is an oddball option, and Gengar's only real way of healing.
Destiny Bond is good for taking opponents down with you.
Mean Look has its uses in-game.
Effective Movesets
1. A doppleganger's doppleganger
- Substitute
- Shadow Ball
- Focus Blast
- Thunderbolt/Hidden Power/Hypnosis
Comments: This set works effectively in the metagame (use Hidden Power Fire if possible there). Substitute is an important move, serving as a buffer against common Pursuit users and allowing you to see what they will use, and/or simply allowing you to pummel one to the ground. Shadow Ball and Focus Blast already have unresisted coverage, so the last move is for more specific targets you want to take out.
2. This doppleganger shares your pain
- Substitute
- Pain Split
- Shadow Ball
- Focus Blast
Item: Life Orb
Comments: Gengar can use Pain Split along with Substitute and the Life Orb recoil to heal itself and beat out certain defensive threats (namely Blissey).
3. Sleepy doppleganger
- Hypnosis
- Shadow Ball
- Focus Blast
- Thunderbolt
4. Did I ever wear scarves?
- Shadow Ball
- Focus Blast
- Thunderbolt
- Trick/Explosion/Hidden Power
Item: Choice Scarf
Note: This Gengar is meant to be a revenge killer against various dangerous threats.
5. Your lead is getting the chills...
- Icy Wind
- Shadow Ball
- Counter
- Focus Blast/Taunt
Item: Focus Sash
Note: If possible, have this Gengar's Defense as low as possible. Counter comes from 3rd-gen; if it is not readily available, take the alternate move from the last slot and use it instead. This set will beat many common leads.
6. MYSTICgar
- Protect
- Shadow Ball
- Focus Blast
- Hidden Power/Thunderbolt/Hypnosis
Item: Life Orb
Note: This is an alternate way of dealing with Pursuit users. Gengar can use Protect to scout for Pursuit, strike back if they use the move, and switch out if they don't.
7. Focus... Punch?
- Substitute
- Shadow Ball
- Focus Punch
- Hypnosis
Note: Gengar might not have a lot of attack power, but Focus Punch is so powerful as it is that it arguably does not need too much. Hypnosis and Substitute are both good ways to fire off unhindered Focus Punches.
8. A Gengar for capturing wandering Legendaries and others
- Hypnosis
- Mean Look
- Night Shade
- Double Team
Note: This Gengar is meant for capturing Pokemon, particularly the wandering Legendaries via Mean Look. Night Shade helps Gengar control the amount of damage it is doing to the opponent.
Closing Notes
Gengar is definitely a top-tier threat. With great power and speed, a number of possible moveset options, and ways of dealing with its worst enemies, Gengar is a monstrosity that is frightening to try to counter.
Future Sight
Gengar did not get much of anything in the new generation, but on the other hand, it does not need much of anything; everything it ever needed is arguably already in its movepool (save for Aura Sphere or something). Gengar is still as dangerous of a threat as ever.