Central American gang violence is pervasive in the daily lives of millions and is well documented. The links below have useful information, but be sure to check the resources under "Where to Start" tab.
Women on the Run is a 2015 UNHCR report specifically discussing the impact of gang violence on women and how it relates to international refugee law.
Gangs in Central America: 2014 Congressional Research Service (CRS) report. Page 3 has gang membership numbers by country.
El Salvador 2015 Crime and Safety Report (U.S. Dept. of State, Office of Diplomatic Security)
This report is produced by a private and public sector group that produces country crime and safety reports.