It is generally legal to customize a Glock pistol in California, as long as the modifications do not result in the weapon becoming illegal under state or federal law. There are some restrictions on certain types of modifications, such as those that convert the firearm into a fully automatic weapon or those that make it more difficult to trace, but most common modifications are allowed.
In California, it is illegal to manufacture, sell, or possess a fully automatic firearm, or any device that is designed to convert a firearm into a fully automatic weapon. It is also illegal to manufacture, sell, or possess a “multiburst trigger activator,” which is defined as any device that allows a firearm to discharge more than one shot with a single pull of the trigger.
In addition, California has laws that regulate the sale and possession of certain types of firearms, such as assault weapons and large capacity magazines. Modifying a Glock pistol in a way that makes it fall under one of these categories could make it illegal to own or possess in the state.
It is important to be aware of and comply with all applicable laws and regulations when customizing a firearm in California. If you have any doubts about whether a particular modification is legal, you should consult with a lawyer or other legal expert before proceeding.