more people die from gang violence than from police
That's an unverifiable guess. No one knows how many people are killed by police every year.
But let's look at what we do know.
The nationalgangcenter.gov site says:
The total number of gang homicides reported by respondents in the NYGS sample averaged nearly 2,000 annually from 2007 to 2012.
The washingtonpost.com article I linked above says:
That number – which only includes self-reported information from about 750 law enforcement agencies – hovers around 400 “justifiable homicides”
The site fivethirtyeight.com investigates a sample and says:
about 1,000 deaths per year.
This guy created a database from all the police killings he could find in 2011. His count is:
police officers in the United States shot 1,146 people, killing 607.
So, with incomplete data, it does seem likely (but uncertain and unverifiable) that "more people die from gang violence than from police." However, the ratio is not terribly high.