Being called to the stand as a gang expert is very different from being called as an investigating officer. As a gang expert, you are not part of the "prosecution team." You are there to assist the "trier of fact," according to California Evidence Code 801.
For those reasons, you shouldn't remain in the courtroom after you testify, even if there's no order excluding witnesses after they testify. You're nonpartisan. You have no stake in the outcome. After you testify, leave the courthouse.
What you do is an art rather than a science, so you must impress the jury with your background, training, and experience.
Richard Valdemar, a retired Los Angeles County Sheriff's Department sergeant and POLICE contributor, has testified as a Mexican Mafia expert in each of the 16 Eme murders my partner Ruth Arvidson and I have prosecuted. As Valdemar says, your knowledge of gangs should come from many sources.
Valdemar started as a 17-year-old running a teen center in Willowbrook surrounded by gang members. Other experience includes growing up with friends or relatives who became gang members or encountering gangsters in military service.
There's more. Describe the courses on gangs you took during your academy training. Describe your exposure to gangsters on assignment in correctional facilities or on patrol duty.
Make notes of all the seminars and classes on gangs you attended after graduating from the academy. List the subject matter and how often you attended or the number of hours. Keep a written log of all the books and magazine articles you've read. As the years pass, your records can be less and less detailed to the point where you won't keep any written records at all and your testimony about your background becomes second nature.
Some gang experts are contacted by defense attorneys and asked to review case material and give an opinion about their client's involvement in gang life. Prosecutors should have no problem with that. The fact that a defense attorney thinks enough of your credibility and expertise to ask you to review the case material in an unrelated case should be brought out during your testimony in the current case.
Finally, as a gang expert you must be proactive, especially with a newer prosecutor. If the prosecutor doesn't ask you about your background, training, and experience before trial, then you should initiate the contact as the trial date approaches. Be sure to volunteer that information.