After a bit of research and transliteration of nib sizes I went ahead and measured a G nib. I drew the widest line possible and measured it with a digital caliper. The results: 1.2 mm wide.
So, I started looking for premade guide sheets for this size and found nothing. Again, taking matters into my own hands I created my own guide sheets in CorelDraw. Three nib widths for the ascender and descender and 2 nib widths for the base for a 3:2:3 ratio. Slant lines are set at 55 degrees All this for Copperplate.
Photo shows the sheet in use, my El-Cheapo oblique holder with Zebra G nib, and my homemade ink well. Ink well is a 3" X 3" X 3" hickory cube cut with a hole for a 3/4" diameter Solo plastic cup with lid.