Ok, thanks for refining the point. Still, there is quite a jump in disposition points from neutral to warm, etc., if you see what I'm saying, so depending on how effective each gift/conversation has been, the gift potency will start to decline eventually, but not per gift given, as seemed to have been the impression.
Each gift given imposes an increasing penalty upon subsequent gifts. So the first gift would give +5, the second would generally give +4, etc.
No, actually, there's a stacking effect.
That is, let's say you give Morrigan a necklace in camp and get a +6. If you immediately give her another necklace/gift, you will get +5…and right after that, the next gift will get +4.
According to the wiki:
When I tried, they gave me an ultimatum - had to choose one.