When you can gift the goddesses and they want your gift, you get 80 points towards their intimacy levels. The sooner you can level each one up, the more attack and defense and magic and skill etc. bonuses you get. So it helps to know which gift works for each goddess. I have some figured out, but others I just don't know. I'll list what I know below. Please help all of us by filling out this knowledge with the answers you have found.
So far I have found that for most, you only give the gift in relation to the right question, only when they first ask that question (but Wang Yi always asks the right question). If you give it any other time, they don't want it, and so you'll have wasted 88 gold. So you answer their question, then give them the gift.
Ruo Yi - When she asks if you know what a certain knot is, you tell her the "true love" knot, and then you can gift that to her.
Guan Yuan Ping - I had found the heart lock gift worked whenever she would say "Your heart only belongs to me, right?" But that stopped working. So now I don't know what works for her.
Cai Yan - When she misses her family, you tell her to write a letter. She then says she needs the "scholar's jewels." That's when you gift her the "Scholar's 4 Jewels."
Xiao Qiao - When she tells you the famous Rouge is on sale, buy it for her.
Wang Yi - Jade bracelet (always asks the right question).
Zhen Ji - Always makes herbal tea. Haven't found a gift that works yet.
Diao Chan - Only two questions so far. Haven't figured out her gift yet either.
That's as many as I have married so far. Will update this list over time.