There's no doubt that the most determined criminals would be able to get hold of guns if they wanted to no matter the laws put in place. But that seems to amount to an argument running along the lines of "we can't stop every gun crime, so why even try to stop any?"
What about, for example, anyone who comes to a relatively spontaneous decision? If you can just walk into a Walmart and buy a gun, that's going to make it a lot easier to get hold of one than if you had to explore the criminal underground, or know the right people, or whatever less above-board routes to get hold of illegal weapons British criminals use.
At any rate, it's pretty clear -- based on the UK and Australia in particular -- that gun laws work for drastically reducing the rates of such crimes. It's impossible to stop them altogether, but preferable to allowing them to continue unabated.
But, no -- already Trump has come out and said that this is not a gun issue, and already many other lawmakers have said the same or very soon will, and after yet another massacre the US wring their hands, pray a little, and then let it happen again.