I found finding a lead in premier difficult. Of course it depends on the rest of your team. You always have to think of what could come, when to switch, to what, do you have a generalist safe switch and so on. Popular leads last season were togekiss, magnezone, Dragonite, sometimes Gyarados , machamp, swampert and garchomp. Metagross a bit more rare because it takes a longer time to charge. That said it is not bad as lead, you can built up some energy and then switch to e.g. a Snorlax. Also you are only forced to switch when encountering a garchomp or rare mud slap rihornior. Magnezone is so-so, cause you are doing not much damage with the fast move and the first charged move - on the opposite magnezone won’t use it’s debuffing wild charge early, or directly switch afterward. Gyarados is similar, it takes some time to charge crunch so you can decide whether to shield that or built energy and then switch out with built up energy. When encountering togekiss you will likely force the opponent to switch to a Metagross itself but because of togekiss in the back you cannot stay in for long then… Or they bring machamp or ground-type or magnezone from the back.
Swampert is of course dangerous for your complete team, at least Gyarados resist it’s moves, but because of not dealing superhigh damage (DB is worse here than WF) it can build up energy against it.
I used a smack down rhyperior with superpower as lead cause it was good vs Dragonite and togekiss and also not bad vs magnezone and snorlax and when facing a Metagross I just could fire off a superpower and then switch to my metagross to catch the meteor mash in time. You should think a similar way with your team; but I don’t have experience with the new Gyarados of course. It is half an all-rounder, but very weak of course to magnezone, so yes, it can lead, but prepare to switch directly to something neat when you see magnezone. When switching to Metagross from gyarados people will likely stay in, baiting or firing off two WC, when switching to machamp, people might switch to togekiss or Dragonite and farm you down. Both situations can be quite dangerous