Gascon used to be district attorney in San Francisco. From Wikipedia:
During Gascon's time as [San Francisco] District Attorney, property crime increased by 49%. Some of Gascon's critics have blamed this increase on his office's reluctance to file charges against low-level offenders; during Gascon's tenure, misdemeanor charges were only filed in 40% of cases presented by the San Francisco Police Department.[26] Having worked with Gascon, San Francisco Mayor London Breed and City Attorney Dennis Herrera declined to endorse him in his bid to become the District Attorney of Los Angeles County; Breed and Herrera instead endorsed his opponent, the incumbent Jackie Lacey
Sounds like his move to L.A. is either a case of failing upwards or an attempt to outrun his bad reputation.