does anyone know what G stans for in Fred G Sanford

The 'G' in Fred G. Sanford stands for NOTHING!

"Fred Sanford. S-A-N-F-O-R-D period," or, "Fred G. Sanford.
The 'G' is for...", with "G" standing for a word applicable to the situation.
It is never revealed what the "G" actually stands for in his name.