My DS1 used to do the same, he could write his name backwards perfectly 'mirrored', I was amazed and so was his teacher, if you held a mirror to it, it would be his name the correct way.
He also had the same problem with 'S' and it was one of the letters in his name.

It just seem to correct itself. We would get him to practice and if it was wrong, ask him if he thought that was the right way. Then get him to write again.

Have a list of words for him to write that have G's and S's in them and get him write them down over and over.