There is no gun show loophole; you (as a private individual, not a gun dealer) can sell your property, including guns to other people who are not known to be barred from owning firearms. There is one gun show a year within two hours of me. The last time I went, every sale at a table required a background check. This isn't state law, it was a policy of the show. I believe there were face-to-face sales, individual to individual as well; this isn't illegal. My state has a very high firearms ownership rate yet one of the lowest murder rates in the US. Coincidence?
More to the point; a background check would not have (and unless I'm mistaken, didn't) stopped the person responsible for this massacre from obtaining weapons. It's unclear why a tragedy such as this brings out all the bleating about background checks for private face-to-face sales. Incredibly restrictive firearms laws didn't prevent the massacre at a concert in France a couple years ago. Restricting people from legally owning guns does not prevent terrorism. The guns are here, they're all over the world. You can wish them away all you want but they aren't going to disappear.