Replicating from standby to standby is called cascaded standby conf.
This is a matter of choice. If you've the required bandwidth, you can directly replicate from primary to multiple standby databases, or if you don't; then you can implement a cascaded conf. There may be other cases where you want to have a cascaded conf. It depends on the context..
And yes, GG can extract data from standby. That is fetching necessary data from a DataGuard standby system. There is a good post about that,
see ->
https://www.ateam-oracle.com/post/oracle-goldengate-best-practices-goldengate-capture-from-a-dataguard-with-cascaded-redo-logs