The above two are correct . 2a and 2b would be correct by adding " to ". For example: 2a. They gave a present to each other. 2b. They gave presents to each other.

1b is the correct one. "They gave each other a present" sounds weird, because how can they give each other one present? If they "gave each other," it means they each gave and received a present, which means there has to be at least two presents.