Trade my Fire Breath Breath Dragonite for a SC Gengar

Seems a worthwhile trade, assuming you will not miss the DB Dragonite. SC Gengar, I'd say, is more valuable.

Personally I’d probably keep it just because it’s a legacy move but you’ll probably get way more use out of SC Gengar, especially if it’s a guaranteed lucky and will have good IVs

Leaving out that you already have a SC Gengar, i wouldn't do this trade.

Shadow Claw Gengar might come back, there could even be a gastly community day in Future. Dragon Breath Dragonite will never come back, not now we have Latios.

Hoesnly I woundn't do it either, SC gengar really isnt worth all the hype, after using one against mewtwo multiple times it has always let me down and its too glassy to be useful not to mention you do already have another SC ganger, that and it could come back Dragon breath dragonite wont come back and it still a very viable attacker, it may not be as good as dragon tail but its far from useless, and with all the dragon types and neutral match ups dragonite gets an advantage in it will be a lot more useful than a shadow claw gengar

There's a lot of hype around SC Gengar but honestly it's mostly only useful for hardcore soloists/shortmanners and you already have one, which is sufficient for most solo uses. I would use your guaranteed lucky trade on something else.

If you aim for trio or help you in 4 man mewtwo, then go ahead. It will be helpfull especially against psycho cut mewtwo. Nice damage bonus give you more ball to catch it, and better reward.
If you ussually raid with a large group of trainer, you dont need it. Even using your dragonite is good enough. Having damage bonus with 10 or more trainer is hard unless you are raiding with a very weak trainer.

SC Gengar is a giant letdown
Don't