Data recovery and Computer forensics isn't about software. I'm kind of amazed your asking this question if your moving into this field.
Having done the course, I'm a bit shocked that people consider they are ready for real world Computer Forensics with only a GCFA.
My advice is don't stop your training there, if your going to sell data recovery as a service, you should learn about it.
What training would you suggest? What more can you learm about Data Recovery? Do you mean hardware and fixing drives, as menioned previously, this is an expensive side of data recovery to get into as often the equipment runs into thousands and thousands to purchase!
There are two related sides to data recovery
The first is physically reading sectors from a disk. On some disks they read 100%, other times a few or very large number are unreadable. The large number indicate that you may be into expensive physical recovery methods, such as head swapping
The second is logically decoding these sectors. This can because sectors are missing (see above) or the file system has been corrupted, partially overwritten, deleted, operating system reloaded, partition sizes changed etc etc.
The training, or learning you require for this is knowledge of file systems. How to logically read a file when lots of useful information is missing. Which tools to use, and knowing when the results are correct.
Start with NTFS and FAT, the starting looking at the various flavours of Unix/Linux. This will keep you busy for a few months.
The next stage of knowledge is file structures. Data carving works for unfragmented files, but to join fragments together is a skilled job.
Never stop leaning!!
As Michael says, learning file system fundementals is a great start. You should have got Brain Carrier's File System Forensic Analysis book with the SAN's 508 course which is a good book to start on (though not referred to enough in the actual course when I did it a few years back).
Youtube can be a great place to look for the hardware side of things ..nothing beats hands on experience with drives/media. Pull apart some old drives, see how they work interally, follow some of the walk throughs for swapping head assembly's and similar techniques that are on the net.
Training courses like the advance X-Ways ones give great file system knowledge also.
In the end, nothing beats hand on work ..test out as many tools and techniques you can.
Good luck!