There are some excellent resources on designing and conducting needs assessments in Chapter 3 of the Community Tool Box at /en/table-of-contents/assessment/assessing-community-n.... We do not have a specific needs assessment that relates to gun violence prevention, but you may be able to develop something useful from the instructions on how to develop needs assessments thee.
You might also want to contact the US Centers for Disease Control and Prevention violence prevention resources at https://www.cdc.gov/violence-prevention/index.html. They should be able to guide you to some recently used and successful approaches.
