I want to fire fuction in every insert to gl_entry

[ejaaz]

IMO, it is related to this?

How can find the code that fire when bulk sales invoice for next status in its workflow

You can define these functions in your custom file by overriding them. https://frappeframework.com/docs/user/en/basics/doctypes/controllers

To run a script on form submission, use the on_submit method.
https://frappeframework.com/docs/user/en/basics/doctypes/controllers