Gyarados

Gyarados with Bite/Crunch will be useful against Mewtwo, Alakazam, Gengar, and Exeggutor, provided Mewtwo does appear the next couple of days. Gyarados can also be very effective against Entei with Hydro Pump (although, I've yet to fight Entei yet lol). Other than that, I rarely use Gyarados for raids.

My Gyarados has Dragon Tail/Hydro Pump, and does very well as a generalist attacker. GamePress lists DT/HP as Gyarados' best moveset, so try that one out and see how it works for you when you're ready to evolve your Magikarp!

Definitely worth powering up! Ideal moveset is Bite / Hydro Pump, but not always easy to get. My 100% currently has Bite / Crunch. I've used 4 Charge TM's on him an he never gets Hydro Pump. Bite / Crunch is good vs. Mewtwo, so I've stopped with that. I already have two other lesser Gyarados with B/HP.

There are few bad movesets on gyarados, which it nice. Getting Bite makes him useful in raids, especially Mewtwo and Alakazam. I used mine in every Lugia raid I did. It is better if it gets hydro pump or crunch to match bite.

Dragon Tail makes him a good general attacker, as has been said. He can get double Dragon attacks without taking SE damage from dragonite. DT/HP would make a good sweeper. DT/Crunch wouldn't really have a great use.

B/HP on Gary is the GOAT

Personally I might power it up...but I would sure stop walking it after you evolve. I'm somewhat running out of things to walk, I still wouldn't continue to do that one. If you are on your first gyarados you must have a dozen other candy needs

Yes, it's quite useful. Just Charge TM Twister, anything else has use for something.

I live in a water biome and catch magikarps constantly so I have 8 gyarados and 3500 more candies to boot. I get a lot of mileage in gyms using my bite/HP versions as they can dish out damage versus most anything. I also have a high IV dragon tail/crunch (14-14-14) and bite/crunch (14-15-15) which are OK general attackers and good dark specialists. .