Are you using Codewarrior? If so, then there's no need to use SRecCvt. CW burner is much better and flexible converting S records. Converted file is stored in project \bin folder with extension *.phy. Perhaps you bumped in to the bug of CW Project generated bbl files for S12G devices?
Please find in your bbl lines below and change destination to 0x28000 (delete one '8').
origin = 0x0A8000
destination = 0x288000
Also find these two and change destination to 0x31400 . Flash at CPU address 0-3FFF in S12G belongs to page 0xC. Destination should be origin + 0xC*0x4000.
origin = 0x1400
destination = 0x001400