Stupid assassins will toss a gun into the water thinking no one will find it. Instead if you put the gun into HF for a few days they will never be able to confirm it is the gun that killed this person. If the gun is kept in long enough it should be able to remove the serial number as well.
In the Godfather Michael is taught to drop the gun immediately. This is because the Mafia can intimidate witnesses to say they don't know who was doing the shooting, and if the gun can't be traced to Michael and they don't catch him with the gun it will be much harder to convict him even if the police were to catch him five minutes later. You still have gun shot residue, but there are ways to sidestep that, especially if he can avoid being apprehended for 24 hours.
So a pro is caught between two key goals, get rid of the gun as soon as possible and really destroy the evidence by dropping it in HF. It would be foolish for him to have stayed in a NYC apartment but it seems he had a bag hidden in Central park. So that bag which probably had a change of clothes could have also had a plastic container with HF in it that he could then drop the gun into, put the lid back on, and then put the whole thing in a dumpster or down a sewer.
So here is the point, if we cannot find this gun it strongly indicates he was a pro.
To me if this were an amateur we would already know his name, the fact that they don't have that, they don't have him and they don't have the gun, this looks like a professional to me.
Also think about the shell casing, he knew the cops would recover that and so the writing on that strongly indicates the man was killed by someone who was stiffed by the insurance company. That is a very clever way to inject "reasonable doubt" if this shooter had no connection to the insurance company and if the reason for the hit was unrelated to that.