Explanation

"The gift was bought for her."

"For" is used to indicate the recipient of the gift or action.

The gift was bought from her.

The correct preposition is "from" when indicating the source or seller.

So, the sentence becomes: "The gift was bought from her."

The gift was bought by her.

The correct preposition is "by" when indicating the agent performing an action (who bought the gift).

So, the sentence becomes: "The gift was bought by her."