It is unknown if this gang existed before the outbreak or if they formed afterwards. However, given the fact that Sofia knew to warn Nick to be careful of them, it can be assumed they existed before the outbreak.
After the infection hit and the civil authorities fell in Baja California, La Manas took advantage of the lawlessness in the state. They began to rob and murder survivors as they roam the highways. Owing to their behavior, and as mentioned by Sofia, they are to be avoided while traveling to Tijuana. Unfortunately, Nick did encounter them and they immediately began to fire upon him. During their second encounter with Nick, two of the La Manas drug runners, including their leader Gael, were killed by a zombie horde on the highway. The third member escaped in the Jeep, leaving his status and location unknown.