Receiving a gift does not obligate you to send gifts to those same givers in the future. A gift is a gift, and should come with no strings attached, however, a thank-you card from the recipient should always be sent to all gift givers. If you receive an invitation to a baby shower and choose to attend, you should bring a gift. If you receive an invitation and choose not to attend, you are not obligated to send a gift, however, you certainly may if you wish.
Again, while gifts are not always required, when it comes to family, I always play it safely. If a family member is celebrating a major event such as an upcoming birth, wedding, milestone anniversary, etc., I always send a gift whether or not I attend, so as to avoid any potential hurt feelings. If my in-law's or their family members gave me gifts, I always tried to return the favor if the opportunity came, as my mother in law wasn't always the easiest person to please, and I didn't want her to feel as if I were snubbing her relatives. Though there were many times when I absolutely was not obligated to send her relatives gifts, I did so anyway just for the sake of keeping peace and making the effort to get along. But this was just my personal rule of thumb, it it certainly doesn't mean you have to do the same. Just use your best judgement.