The Tavern

The effect of GDR is rather significant in protecting from melee damage (e.g. see Berder's graphs).

Rationale has been given in the past for not displaying this value, not disclosing its existence in game, and not informing players of it in the forums: that sources of GDR tend to also be sources of a lot of AC, so that the two figures are somewhat correlated, and that the devs intend that players should intuit the combined effect of AC+GDR as a function of the displayed AC.

(along with the implied acceptance that it's fine to thoroughly mislead people about the situations when they aren't well-correlated, such as octopodes getting their AC from rings)

I propose making GDR purely a function of AC, rather than being derived from your choice of body armor, so that the reality truly is that damage reduction is a function of AC.

Also Stoneskin, Ozo's Armour and Cigotuvi's Embrace would affect GDR then, I assume?

Yeah, that's what I had in mind, so that one's AC total from using these spells isn't less effective than one is led to believe from prior experience. This would be a buff to these spells, but I don't imagine it would make them overpowered. If really needed it could be toned down or even cancelled out by reducing their AC gain, resulting in a small nerf against non-melee attacks.

I mean those spells seriously need balance changes already, so making it slightly more urgent wouldn't really hurt much.