the best gems producer is Money Badger : 2 gems per day at level 15. (limit : 2 money badgers per land, i got 2)
then you have the Golden Retriever : about 1,3 gems per day at level15
they were breedable , but s8 removed them from the market.
i doubt they would come back
all the other treasure animals produce 1 gem per day which is very low
btw you got gems the battlegrounds and also from the medal icon under the task icon (top left)