GDFS In Your Phone Is Like Bios In Your Computer.
You Cant Backup You FS And Main But You May Check It And Download Same From Download Area. In Standby Press >*<<*<*. First Line Is Your Software Version While In 2nd Line After CXCxxxxxx You'll See Your MAIN Name. Now, Go To Service Info And Then Software Info. Under Customization Read 4th Line Which Will Be Your Customization. Use It To Find Your FS Here. However If You Want To Backup Your Data Including Contacts, Messages, Notes, Calender And Files etc, You May Use MyPhoneExplorer To Do So.
Here Is A Guide For Flashing Using XS++, Backing Up GDFS And Recovering From A Bad Flash. See XS++ 3.1 Guide-Flashing,Customizing,Recovering - Top Sony Forums