I wrote a matlab script to extract some channels from the RAF files, but the g forces always seem to be zero. I checked it in a binary editor and it's the same there. Is there a way to have this data in the raf files, or another easy way to construct it from the other data?

Try RAFs downloaded from Hotlap Analyser http://www.lfsworld.net/rafa/ . I think these G's were added in connection with Hotlap Analyser development and are visible in it, so the values must be there. They were not available in older versions of RAF specification. Maybe it's a bug if they are not present in regular RAFs output from LFS (I haven't checked that) and maybe your RAFs are from older LFS version (?). G's are accelarations, so another way of getting them is calculating 2nd derivative of car position and doing coordinate transformation if you want longitudinal/lateral parts.