Attacked Scalding Rock Elemental using only autoattacks. They are all level 80, so stats should be consistent.
Test 1: I allowed the elemental to proximity-aggro me, waited for Gift of Arthas to proc, then began autoattacking
Test 2: I clicked off the Gift of Arthas self-buff, then simply found another elemental and began autoattacking
The results were:
Test 1 (Curse applied): 503 autoattack Hits, 50 minimum damage, 52 maximum damage, 51 average damage
Test 2 (no Curse): 527 autoattack Hits, 44 minimum damage, 46 maximum damage, 45 average damage
This seems to be a damage bonus of only +6. However, I believe Gift is applied before Armor reduction (contrary to other comments on Wowhead), based on this evidence:
Expected unmitigated damage based on the character sheet should be 60-63, or 61.5 average damage
Without the Curse active, actual damage was 44-46, average 45, which is 73.17% expected damage.
Assuming the Curse's effect is applied additively and before Armor, expected average damage would be min 68, max 71, average 69.5.
73.17% of 69.5 is 50.85, which is extremely close to the produced results of 51 average damage against an elemental with the Curse active.