The SANS forensics courses are good. There are more of them than just GCFA as well - go check out the SANs website for more information. There are new streams in network forensics and some more general purpose beginner certs now as well. SANs also have GCIH (Incident Response and Hack tools) and GCIA (Intrusion Analysis) which I would highly recommend. If you want to do computer forensics seriously you probably want to take a course that covers the presenting evidence in court part of the process. If presenting evidence in court does not appeal to you (it doesn't to me) you may want to refocus more on the incident responder area rather than the computer forensics examiner area. There is some skills overlap, incident responders do need forensics skills.
As for the pen testing courses/certifications you mentioned I have the GPEN and OSCP, and while both were great I can say that both had very little to do with forensics or the investigative side of the field. There are better things you can take to learn to think like an attacker and defend against them, GCIH would be a prime example.
Feel free to ask more questions if required.