Hmmm... I'll try to give a stab at making a simple guide to the Gearing. Now I am NOT the best with this ship, but I'll write the things I have learned while playing this ship. Feel free to disagree with me.
First, this is a straight up lie in the game. Don't believe it. The Gearing's guns are good, but they got power crept hard. Do NOT engage with other gunboat destroyers (Harugumo, Khaba, Kita, maybe daring) unless you have a CLEAR advantage. And I put emphasis on the CLEAR CLEAR advantage. Like make sure that one of the following is true before engaging: have a friendly nearby cruiser or a friendly dd (make sure the dd can shoot. I tend to hesitate if a shima is the dd close to me because of the bad rep that shima have for not using their freaking guns) close to you that also have a clear line of sight on the enemy dd, you have a massive health advantage, or you are in smoke and you can shoot the enemy dd without risk of counter fire (watch out for counter torps though).
Yes, people will tell you that Gearing is excellent on close combat because then the horrible firing arcs will be neutralized and the fast reload can be made excellent use of... In my experience, this is not completely true. You might barely win against a khaba at super close range, but you will probably get torped (the gearing no longer has the fletcher maneuverability) or you will barely survive with little health left. Similar with picking a fight with the daring, but the daring has the repair party so it will probably out health you. The harugumo... forget about it. So, the golden rule with the gearing is PICK YOUR FIGHTS. Pay very close attention to the mini map and the health of the enemy dds.
So yeah. Since the guns are NOT superior (they really need to change that. Might be true historically but not in this game) to most of the dds in the game, be very careful with fighting other dds. As a result, do not contest caps willy-nilly with the Gearing. It is no longer an amazing cap contester. It has been relegated to decent cap contester now. So, in the beginning of the battle, don't just rush into a cap. Look at the mini map, make sure you have enough support. Don't be afraid to let the enemy take the cap first, you can always flip it again. Don't be afraid to use islands. Poke your head out of islands to scout, duck behind them if things get rough, play ring around the rosy with them. Your arcs should be able to fire over islands. DON'T ISLAND CAMP THOUGH. You are a dd with decent concealment. Use it. Every time you decide to cap, make sure that you have an escape plan at the ready. For example, sometimes, i run into an island near the cap and enter the cap [edited]first so that i can book it if a dd pops up that i cant gunboat down.
Now, the huge advantage the Gearing has is its super long American smoke. So you can always smoke up and start shooting. or smoke up teammates, but that is difficult in solo randoms because you don't know if the guy is going to use it. However, with radar, and smoke being a huge torp magnet be careful if you stay inside smoke and start shooting at enemy ships. As a result, if you do decide to smoke up and shoot, look at the minimap, take note of any potential enemy dds and radars, make sure team can spot for you, and make sure that the ship you are going to shoot at has enough health to make it worthwhile (like no need to smoke up and shoot at a ship that is alone with zero support and has like 1000 health. Just open water gunboat that ship down).
The torps are workable. Now that WG gave the Gearing the fletcher torps, the Gearing is a proper upgrade from the fletcher. Just use the torps similar to how you used them in the fletcher. The 16 km torps are also useable, but if you do decide to use them, be warned that they have a ridiculously long reload so you won't be able to use them as often as well as lower damage. As a result, you can no longer spam them like the fletcher. I'm not a good torpedo user (I SUCK at the japanese torpedo boats) so I really shouldn't be giving advice on how to use the Gearing torps.
Now, I do struggle with playing the Gearing solo. It is not a great solo ship for me. It is a beast if you play it with a division though. It is because of the smoke and workable concealment. You can smoke up div cruisers or you can contest caps with div dds much more easily. The Gearing is a huge multiplicative force in a division. There is a reason why Worcester, Mino, and Gearing in one division is just pure cancer. (I haven't done this though. I have played Gearing, Harugumo, and z-52 in one division. Powerful guns, fast torps, hydro, long smoke all in one division. It was hilarious and powerful.)
Captain skills and what upgrades... hmmm... I don't think I can do this one. Even I'm not sure about my build. I just maxed out concealment and got faster torps. I have survivability expert because having more health than the enemy dd is a crucial factor for engaging in gun fights with gearing. I also have rpf for obvious reasons. Flamu already made a good video on youtube about it. Legendary upgrade on the gearing is worth it from what people tell me. You get detection that rival shima after all at the cost of slightly longer torp and gun reload.
So yeah. Here is my super simple guide to the Gearing. In conclusion, it is a classic example of the American line's niche. It is a jack-of-all-trades. I don't recommend this dd if you are a solo player. If you division up a lot, the Gearing is a great... no, it is an amazing ship to pair up with cruisers or other dds. Not really battleships as much since they can't use your smoke as effectively. As a result of the OP smoke, good concealment, and workable guns and torps, it is an amazing ship for clan battles. For ranked.... workable