Generally people post the major bits of their build, but there really are no minimums or maximums, just what you feel needs showing.
I find GBs fall into one of three types,
Historical where many builders give a lot of history about the subject they are building (WW2 carrier based planes, Mig Killers, supersonic bombers etc).
Technique where the focus tends to be tips and tricks of a particular kit or method of building / painting. This could be a particular kit (Revell 1/72 P40 Warhawk), a type of kit (resin, vacform etc) or technique (natural metal finish, splinter camoflage etc).
Just for fun, what ifs, sci fi, story based (WW3) etc. Really all GBs should be fun, but these seem to be more about the set up, and less about the building.
Of course most GBs include some of all of these bits, its really more where the focus of teh build lies.
As a natural metal finish is basically about different was of doing a natural metal finish, you can kind of think of it like writing a how to article. Show off how you are getting your finish, and offer the good / bad to the particular method you have chosen.