Reasons this

is dubious:
... a nuisance to those of us there to work out, mainly because they're loud (always yelling) ...
IRL, gyms like 24 Hour (where I've been a member) have semi-isolated rooms for their basketball court(s) (at my gym it was a basement room). It keeps stray basketballs from rolling into other workout areas.
IRL, gyms have loud background music to motivate/propel workouts and drown out Jim Guy's grunts and the thuds and clangs from his weights workouts (some of which are proper safe form).
... numerous (they take up all the parking places ...) ...
Your city doesn't mandate parking lot size according to building capacity?

So ... two half-court games with 3-4 players per team and a third group waiting on a court? Even at a car for every player, that claim is

-grade BS.
... drift out into the workout area and mill around on the machines you're trying to use ...
If only for the sake of safety, a decently laid out gym has wide aisles between rows of machines and even wider walkways between areas for different types of workouts.
Speaking of safety, is
DU-member ChoppinBroccoli really claiming that basketball players are so clue-free about their own and others' safety as to wander among cardio and weights machines?

... of the typically hot-headed late teens/early 20s age demographic. There are always fights and altercations, etc. ...

Stereotype much,
DU-member ChoppinBroccoli?

What semi-decently run gym doesn't shutdown groups that allow fights and give members who actually fight the dis-membership heave-ho?
The 19 year old pulled a gun OUT OF HIS POCKET ...

Whether this

is fiction or happened,
DU-member ChoppinBroccoli doesn't know Ohio state gun laws regarding handguns,
https://codes.ohio.gov/ohio-revised-code/section-2923.211 :
(B) No person under twenty-one years of age shall purchase or attempt to purchase a handgun, provided that this division does not apply to the purchase or attempted purchase of a handgun by a person eighteen years of age or older and under twenty-one years of age if either of the following apply:
(1. LEO
2. Military service)
The 19YO, if he and the gun really exist, may have gotten his gun easily, but he did not do so legally.

So, the 19YO was playing basketball while he had a handgun in his pants or shorts pocket?

... shot the 23 year old point blank ...

Other than cheesy detective novels and melodrama-queens, who talks that way?

... ran out the back door.
Gym members have direct access to uncontrolled back/employee entrances? Not at gyms I've used. Gyms have a single entrance monitored/controlled to limit use to paying members.
To be sure, none of these are
not-even-in-an-alternate-universe impossible, but each is implausible or worse individually, and each implausible claim compounds the next. IMO, this bouncy is fiction, and not very well composed fiction.