MAngband

This spell doesn't exist in Angband 3.0.x anymore. I understand that in turn based this is not really needed, since you have time between each action to plan carefully your next move... but in real time this sounds like the end of the mage class. Basically, instead of playing warriors, mages, rangers, rogues... you will play warrior with scrolls, warrior with some spells, warrior with other spells. Mages without GoI would have to sacrifice INT for CON... playing a mage without GoI with 600 or so HPs would be totally suicidal, with max CON however, a mage can still get around 900 HPs (high elf - would be higher for exotic combos like HT mage), but then good luck to maximize your INT too. I tend to think that a form of protective shield should be kept for mages at high level... but probably not the current GoI (too powerful IMHO).
For PWMAng, I've implemented a GoI that is available sooner (lvl 35 instead of 45)... but that only absorbs (charlvl)% of the damage. This is more than enough for mages.
The only problem with Angband 3.0.x is... mage spells are limited to 64 spells, and there are already 64 spells implemented! So keeping GoI would mean removing one of the other spells... (PWMAng doesn't have that problem, since rogues get a separate set of books/spells, so the specific rogue spells like Detect Treasures are free slots for mages).

Absorbing a certain percent of the damage from all sources might be no different than giving mages more HP. How about one that absorbs all damage from ranged attacks, such as breaths and spells? It might result in one or more of the following:

No more instadeaths from double breaths and high uniques that cast/breathe for more damage than mages have HP.

Melee is still a threat, so mages might prefer to stand back and shoot rather than melee.

No longer "cheating" to use GOI during PVP.

Completely break Vanilla canon.

This is a nice idea... I'm gonna restrict GoI in PWMAng to non-physical attacks (basically spells and breaths). Should help mages a bit without being too cheezy.