The only thing needed to fix Gungnir as a weapon is to make it such that def down procs never overwrite a stronger def down effect (this should apply to all forms of def down; Acid Bolts should not overwrite Angon, basic merit Angon should not overwrite relic+2 Angon, etc).
The relic weaponskill could use an overhaul, but that could be said of almost all relic weaponskills, so is a completely separate issue.
The aftermath is largely irrelevant since you'll never use the relic weaponskill. Until the relic weaponskills are overhauled, there's no reason to consider any aftermath changes.
The problems with drg's damage are due to two issues:
1) The inability to get both the attack+DA of /war and the haste+store tp of /sam. All three other 2-hander jobs (war, drk, sam) get both benefits (or basically equivalent, in the case of drk/war). While this makes drg the weakest of the 2-hander jobs, it's better to say that it puts drg in amongst all the other DD jobs in the game (and probably more at the lead of those other jobs), and that it's a flaw in allowing just those 3 jobs to get those doubled-up benefits.
2) The horrible weaknesses of the wyvern in high-level content (mainly breath attacks and the usual problems of buffing pets).
Neither of those are the fault of Gungnir, which is itself a solidly respectable weapon. If you factored out the double benefits those other 2-handed jobs get, a Gungnir drg would be roughly on par with them.