what is use if complex gateway?

This type of gateway allows you to selectively accept (or restrict) incoming paths and evaluate rules to determine outgoing paths. For example, you can restrict the node to accept only the first three out of four incoming paths, or require input from certain nodes before continuing. Outgoing paths can be configured in the same manner as other gateway nodes.

Looping flow should not be directed into a Complex gateway itself, but into another upstream activity.
When multiple flows enter a complex gateway, it allows only the first flow through, despite allowing you to configure it to wait for a configurable number of instance tokens to arrive before activating. Then, the gateway waits until all other incoming flows arrive before activating the outgoing flow again.

It was there in document whatever you sent

I need real time example

One basic Example:
Assume you have 20 constants you want to update some of those based on the value changes from DB. You can use complex gateway to identify the paths it need to take rather than sending to all the nodes.

Please find the below example for complex Gateway

Eg:
Loan Request Process
An employee request a loan from company. This must be approved by him or
his boss the financial area or the vice-presidency. when at least two of three
approve the request, the money will be given to the employee.

In this situation we have to use the complex gateway as we need to synchronize min of 2 or 3 approvals.
If we do not use complex gateway then for every each approval employee will get lone each time as we are missing synchronize of min of two or three approvals.