I see your profile says you’re high gold currently. I’m high plat, just for posterity so you know I’m not some pro, but also as I’m closer to you, maybe it will be more relevant than high rank advice. I have 100+ hours on Zarya and she’s my second most played. Also most of my playtime is from comp.
My general rule of thumb is; if someone is taking damage, including you. Unless it’s high burst or a headshot, you can see someone being chipped and using a bubble will give you charge and also save them. Sometimes you can do this preemptively (for example tell your Rein you have a bubble so he can charge in or at least lower his shield and you give him a bubble). Most enemies will shoot at it, especially projectile dps. If the enemy has a Junkrat or Hanzo or Pharah, that’s where the real charge comes from. Also, since Rein is attached to his barrier, giving him a bubble while he’s still holding it can still give you charge if the enemies shoot at the center of the barrier (that is covered by the bubble).
Sometimes if you know there’s an enemy such as a Roadhog, you can pay attention and keep your projected bubble to save someone before or after they’re hooked. It can also save a single teammate from ults such as RIP-Tire, Deadeye and D.va’s mech. Saving someone (and yourself) from a tire or Deadeye can be tricky because it depends on when the enemy will activate their ability, but it can be done with some practice. You can protect a Mercy while she’s using rez (guaranteed charge if she wants to rez an important target in the middle of the fight). You can also body block with a bubble and block an Earthshatter for example. If you use a bubble, a teammate can stand behind you and not get hit by the shatter (or even a D.va mech). I also often use my bubble and walk right into an enemy ultimate just to get charge (walking into Hanzo’s dragons is really fun). You have to be careful not to stand in it too long though. You’ll learn to identify these situations as you play.
Basically, this is your “tanking” ability to protect yourself and the team, but it also gives you dps. Don’t waste it too much just in case. It will happen here and there, but if you’re not getting or maintaining charge, it’s usually an indicator that your bubble usage needs work.
Honestly, to me, it’s the same. The only thing that matters to me personally is our team comp. You need a second tank. Considering your rank, any other tank will do, but the best synergy is with Rein (you can sometimes pull it off as a solo tank. Been there, our ranks are weird). I also like having a D.va or a Hog, even a Winston. Winston and D.va are always targeted when they dive so that’s guaranteed charge and also allows them to survive (don’t bubble instantly as they jump in, wait for them to be chipped first). Hog is a big juicy target that has no self shields so he’s often targeted and bubbling him is a good option, especially if he’s ulting (since he’s vulnerable). Be careful with D.va’s matrix as it negates damage done to the bubble (I mostly play Zarya/D.va with my D.va main friend but we practiced synergy). Winston and Orisa aren’t attached to their barriers so giving them a bubble can be a waste if they retreat behind barriers.
I’d say I like her a bit more on attack because you can be aggressive and push in. If your grav is being used well and coordinated, each time you use it, that’s more progress on the payload or more objective % captured.
This definitely depends on what is happening in any given moment. Aggressive when you’re backed up by your team AND you’re over 70 charge. If you’re alone, back away. Dying resets your charge; by backing away you can keep some of it. Also, being too defensive will not allow you to maintain charge which isn’t good either. Remember, Zarya has situations where she isn’t very good. If the enemy doesn’t have a lot of high damage heroes or they’re too spread out, it might be very difficult to do anything with her. It’s not necessarily a fault with your play. Zarya requires a lot of game sense and awareness and is honestly one of the best heroes to learn that with.
Whenever you know that your team is with you and/or your dps has a complimentary ultimate. Hanzo, Pharah, Genji, Reaper and Tracer are the best to me. Deadeye and Tactical Visor can be a wipe if the enemy team doesn’t have a barrier and/or matrix. EMP can help on its own too, especially if you’re high charge. With no way for the enemies to use abilities, you can kill everyone in grav alone if you’re 70+ charge. Even healers like Ana and Moira can help a lot. Coalescence is really oppressive to people in the grav because it goes through barriers and matrix. Ana’s anti-heal grenade can entirely negate their defensive ultimates.
If you’re high charge, use your secondary fire and just lob those high charge bombs into it. Do that anyway just to help, but especially if you’re high charge. Of course, enemy healers can negate any damage in the graviton. Transcendence and Sound Barrier are the best, tho Lucio could be interrupted while casting. A Moira healing orb + Coalescence can save people in it too. Ana’s healing grenade can help as well and so can Valkyrie. It’s not your job to hunt healers around the map, but you can pay attention to them and track their ultimate usage. You can also outright say “I’ll grav when someone kills Zen” to make sure everybody is aware of this issue. Try not to grav in the exact moment you hear Trans or SB being used. They last 6 seconds so wait it out.
Try also not to hold gravs forever. Yes, they take a long time to charge and you might feel afraid of using it, but don’t worry about it too much. Especially while you’re just learning. Generally try keeping one for the final push or final defense, but don’t die and end the match without having used it. It will happen, but make sure it doesn’t happen too often. Although it’s the best to pair it with a dps ultimate, if your dps is down but the enemy is also lacking members (for example it’s 4v4), you could still use it and the teammates that are with you can most certainly help. It depends on each individual situation however.
Also don’t be afraid of using it to prevent something. I’ve solo grav’d nanoboosted Genjis that just deployed Dragonblade since he can’t move out of it. It’s absolutely not a waste to save your team from a potential teamwipe by rendering the Genji immobile. I’ve solo grav’d Lucios running back to the point to contest just outside the objective. You can grav to counter the enemy grav (if you’re both stuck in gravs, you have limited abilities) or other ultimates. If your team is shattered, you can trap Rein in a grav with his team so they can’t rush into your immobile teammates. Again, you’ll learn as you play. Don’t be afraid to test it (though start in quick play).
Never use it if you’re the only one alive though. If you’re the last member of your team alive and you have grav, don’t use it. Unless it’s literally the end of the game and it’s overtime, might as well try, but otherwise, don’t. If there’s time for another fight, keep it.
Sometimes you won’t do well. It’s often tied to your own and the enemy team comp. Sometimes there’s simply no synergy. Zarya is a very teamplay dependent hero. She excels only as much as her team does. If you have good synergy with your team and the enemy team comp is particularly good for Zarya to fight, you might be able to utilise her to completely roll them over. No better feeling than building a grav in 40 seconds or less. But sometimes it will suck and you’ll likely have to swap. I’d suggest to keep trying while you’re practicing just so you learn to tell the difference between no synergy and you simply not performing well yourself.
Lengthy post, oops! Anyway, I definitely recommend you do what the poster above me said as well; look for some guides and coaching. I hope this helps a little bit.