If you don't go you will be arrested, put in custody for a few days and then brought to court - to testify.
Share your concerns with the DA. There is a Victim's Bill of Rights in the California Constitution now which requires the DA to be solicitous of your safety. Remind him/her of it. That doesn't me you won't have to testify, but you may be able to be escorted by law enforcement, and testify behind a screen if identification of the gang member can be obtained in another way.
I've indicated agreement with Attorney Gutekunst's response and defer to the opinions of my learned CA colleagues on CA law and procedure. My reason for interjecting myself into this discussion is to question the adequacy of service of a subpoena by fax to witness' place of employment. The would not be valid service here in PA, and the witness would accordingly not be under a legitimate order to attend. Just wondering if it's adequate in CA. You (questioner) might want to talk to a qualified criminal defense attorney in your area. Most of us offer free, if limited, initial consultations. Take advantage of the opportunity to gain knowledge and understanding as well as prospects for the job of assisting you if that seems advantageous.
I doubt a subpoena by fax is legitimate. My judges always want to see proof of personal service before they will issue a bench warrant. If a subpoena is served by fax, how is the Court to know that the witness received it? If you call the DA then you are ackowledging you got the subpoena. Really need to talk to a criminal defense aore deciding what to do.