Gang activity is often concentrated in clusters in highly populated cities. This increases the recorded numbers of reported crimes exponentially. A great deal of the United States is rural, where population numbers are small. Even considering rural towns have crime, they simply do not have high enough population to count for a majority of crimes.
I do not have any big gang issues around my part of the US but I do believe in other states and bigger cities that gangs are a real problem and do cause a majority of the problems. It might not be something simple like they are doing random shootings, but it could be like innocent people getting caught in the crossfire.
I believe that the percentage of 80 percent seems rather inflated and inaccurate. It could just be that the law enforcement investigators responsible for this figure are being lazy and would rather have us all believe that it's only the "hoodlums" of society committing all the crimes rather than the vast amounts of white collar crime and domestic violence taking place in society.
Authorities are known to use fear mongering and propagating inaccurate data for their own advantage. For instance, police departments like the LAPD are known to fabricate gang data so these statistics are suspect. Data should only be deemed accurate if it's been cross examined and approved by third party institutions.
I believe the term gang is highly overused in the United States when taking into consideration the real influence of gang activity, compared to what is happening in the United States. While a lot of crime can be attributed to gang-related activity, just as much can be contributed to other social problems, so I find the figure highly inflated.