Hi, just a wandering
Fate fan and
F/GO player and couldn't help but attempt to offer some assistance...
In standard story material, I think generally you could think of the
Golden Rule skill as a person's ability/luck of acquiring wealth. The best well-known example being
Gilgamesh of the
Fate franchise (with the highest rank at A), who had no money troubles in his life and is also reinforced by the amassed wealth/treasury of
Noble Phantasms during this period, which justifies the contents of his
Gate of Babylon Noble Phantasm. According to a TYPE-MOON wiki article, it seems only
Siegfried has the
Golden Rule skill out of
Fate/Apocrypha, albeit at rank C- in comparison. There are also a few other Servants with this skill and are mentioned in the reference link at the end of this comment.
In terms of gameplay mechanics on
Fate/Grand Order,
Golden Rule skills increase the NP Gain rate of a Servant for several turns, meaning they can build more NP Gauge per hit (attacking or taking damage) whilst it is active.
For further reading and/or interest, try the
TYPE-MOON fandom wiki for fairly well-documented data on TYPE-MOON material in general or the
Fate/Grand Order fandom wiki for information concerning the mobile game. I recommend them for any fan of the material:
Golden Rule (TYPE-MOON)Golden Rule (F/GO)Hope this helps in answering your question and possibly any others.