I tried to figure out the California Economy organization at one point, and to be honest I didn't get very far. Its usually pretty easy.
In every state I have ever looked at the largest contributor to GDP is always about moving money around - finance, insurance, leasing, real estate (renting, not building), etc. Its usually the biggest piece by far. Then you usually have govco and professional services (accountants, lawyers, etc).
After all that you get into the things that I actually care about - like manufacturing or retail sales or whatever. In California once you got past Ag, Big Tech, and Hollywood - it got pretty murky actually. Of course those three are pretty big.
GDP is really just an estimate, and the BEA process uses primarily labor income and business taxes to estimate this. So when companies like Chevron move corporate to Texas, a lot of that estimated GDP (financing, renting, insuring, etc) will go to Texas. Things can change quickly - its just a balance sheet really, not real life.
Still, I doubt they will be threatening Mississippi for being the lowest per capita GDP state anytime soon.