What is the #1 reason for homicides: the availability of weapons, the attraction to gangs, violent games, or violent movies?

Depending on who you talk to, all of these options are responsible for homicides. I would argue that guns, gangs, violent digital games, and violent movies make for a dangerous combination when it comes to homicidal crime. Let's discuss the options you present.
The availability of weapons is definitely a hot-button issue. Supporters of gun laws argue that it is too easy for potential criminals and terrorists to obtain weapons. Criminals often acquire these weapons on the black market, making it difficult for law-enforcement to track such exchanges. On the other hand, proponents of the Second Amendment argue that law-abiding citizens deserve the right to protect themselves against such criminals. According to the FBI, there were 15, 696 murders in 2015, and firearms were used in 71.5 percent of these murders. 
In 1994, President Bill Clinton signed an assault weapons ban into law, but it did little to prevent gun-related homicides. This is because most murders were committed by those who used handguns, weapons that many Americans associated with self-defense. According to the New York Times, ISIS terrorists have begun to exploit American gun laws to obtain weapons. The group simply sends proxies with no prior criminal records to purchase guns and weapons, making it difficult for law enforcement to distinguish between legitimate purchases and illegal activity. 
Meanwhile, other experts point to gang violence and the prevalence of violent games and movies for the high homicide rates in many of America's major cities (New York City, Los Angeles, Chicago). The National Gang Center reports that the majority of gang-related homicides between 2007 and 2012 occurred in those cities (nearly 67%). Similarly, the Los Angeles Times reports that homicides increased by 27.5% in 2016, and more than half of those homicides were believed to be gang-related. 
As for violent movies and digital games, Psychology Today reports that there are few statistics to back up the claim that violent media leads to higher rates of homicides. That said, some studies have found that criminals who are drawn to violent games and movies are already naturally predisposed towards aggression. In many cases, these criminals grew up in abusive households, where domestic violence was prevalent. Here's an article of interest from CBS that explores whether there is a direct link between violent media and homicidal acts: Questioning the role of media violence in violent acts.
From the above information, we can see that the key element linking all the options you provide is personal inclination. As for which of the options is the #1 reason for homicides, you may have to decide that based on your personal conclusions.