As Tim hints, adding 1P (or 2P or whatever) before the G edit descriptor gives you control of the number of digits to the left of the radix point. There is no S or N form of G. You will want to make sure that you reset to 0P before using any F edit descriptors.
Thank Steve, but using the P edit descriptor with G does not do what I requested. I tried it 1PG13.5 for 55.0 example gives a formatted string of 55.00000E+00 not 55.00000. I can get round by coding but the reason for this post was primarily to suggest that the GN and GS format descriptors would be nice to have as part of the language.