They have all kinds of time because they don't have jobs, don't support their kids, don't have family obligations, or any other responsibilities
Some of that is true. Some isn't. The ones I have worked with have frequently been up to 3rd generation gang members. Most of them don't work, and it's true that they have way too much time on their hands. Many don't have family obligations or take care of their kids, but many do. Particularly with the Asian and Latino gangs here, the gangs have become a support network of sorts for people with few family ties, and/or family that cannot support themselves, like infirm, elderly, and/or young children. The gangbangers literally eat Thanksgiving dinner together, because they have nobody else, and they bring their dependents. It's not just the lure of easy money, drugs, and sex that helps get the teenagers jumped in. It's frequently the lure of a support network (granted, of people you can't really trust, but their own families are frequently worse, at least on the surface) that is what ultimately gets them off the straight path.
Of course, the ones I deal with are mostly from established local neighborhood gangs that are involved mostly in low-level dope dealing and petty theft. If you are talking about higher-end drug cartels like the Sinaloa guys, or "The Family", you are talking about guys who are much more serious about their violence because they are dealing with multi-million or even billion dollar operations. So, it largely depends on how you define "average gangbanger". Local neighborhood gangs don't do much training, as far as I can tell. It's the large syndicates that are dealing in major drug trafficking that are into that.
And whoever it was that mentioned the Yakuza, When I lived in central Japan, there was ONE Yakuza killing involving a firearm in public in the prefecture (population 1 million) in the three years I was there. It was major news. They neither own many, nor practice much with their weapons. The weapons they have are most likely mainly shotguns stolen from farmers (loaded with birdshot), and .38 revolvers stolen from cops. They are not really a first class crime syndicate, and what few modern weapons they have they aren't going to advertise by practicing with them. Not really any place to do it, as even the most remote parts of the country are close enough that people would hear it and report it.