Gengar/Focus Blast far more useful, Not according to GP?

SB Mewtwo will be a strong counter against Mewtwo (since it will no longer have SB) as well as Deoxys. Almost seems not fair - suppose you could raid Rayquazas knowing they won't have dragon moves. Your dragon move Rayquaza would just stomp it. Same thing here - if you have SB, you're golden, and if not, too bad.

“Optimal” here is in regards to raid battles. Shadow Claw/Shadow Ball Gengar offers the fastest energy gain and highest DPS to targets weak to Ghost and quite a bit of damage to neutral targets as well.

Shadow Claw could work well with Focus Blast because it would gain energy faster, allowing it to be used more often before Gengar faints. Hex is slower, so you’d end up doing less damage overall.

Common sights in gyms, i.e big normal types, highly resist ghost type moves, which means in order to fight gyms with Gengar you’re wasting your fast moves dealing little damage and relying on a one bar charged move to deal damage.

In raids, you’d be better off using actual fighting types to do fighting damage (except extreme circumstances), because you get STAB and shared typing with fast and charged moves.

Its obvious, if you go to gyms, you are gonna face Blissey, Snorlax, Chansey, which resist Hex and Shadow Ball but are weak to Focus Blast.
Yes, Gengar with FB will deal massive damage to them, but will also take supereffective damage from their Psychic moves, and he is very frail. Thats why Gamepress ranked FB so bad in comparison to SB.
Mewtwo is better here because he resists Psychic and still hits with neutral stab, so get a FB Mewtwo, use it and compare it to your FB Gengar.

For FB Gengar, the only viable fast move is Sucker Punch since it is not ressisted by normal types.
Bear in mind that the autoselect algorithm is far from optimal and I don't recommend using it as a guideline for good counters. you would be better off taking advice from this site, rather than the autoselect.

I find SP/FB Gengars great for normal stacked gyms - yes it's a glass cannon, but low potion use and quick,

First of all, like other said, if you're going to use Gengar against normal targets, at least une Sucker Punch as a fast move, Hex is doubly resisted and thus you'll be losing too much DPS.

I also wanted to use FB Gengar for gyms. It's fun, it's uncommon, just like PC/FB Alakazam, but in the end it's just less efficient than Machamp/Hariyama. I got a bit disappointed. FB being 1-bar doesn't help.

Also, check out Gampress' comprehensive DPS rankings page. It shows that SP/FB is the second best moveset against Blissey, the first being... SP/Sludge Bomb. The 2 are very close. Sets with Hex deal 2 less DPS. And since cloudy weather boosts both fighting and poison, there is no situation in which FB overcomes Sludge Bomb. Really too bad.