How much is appropriate to contribute to the office gift card pool when a colleague is expecting a new baby? Does it make a difference what your professional relationship is, and how much you like this person? (In this case, it's the head of our department, whom I adore.)
Until very recently, I was under the naive impression that people went to work to make money for themselves and their families. Based on your question and the recent one about fundraising at work, it's become clear to me that companies keep employees around in order to have a built-in cadre of people to ask for money who feel as though they don't have the option to say no. Sheesh. I don't mean to sound like such a curmudgeon, but there's got to be some breaking point where you're allowed to go to work without feeling like you have to hide your wallet.
However, you say you actually like the person, and your question isn't about whether to give, it's about how much. Whoever is spearheading the collection should be able to provide some guidelines. You could ask, "What is the average amount people have been giving?" or "What range of contribution are you looking for?" If you don't get any direction, think about what you spend if you were buying this person a gift on your own. For a baby shower at work, I would guess that the $10-20 range would be appropriate. If a large number of people are contributing, it's possible that as little as $5 from everyone will still provide a nice gift.
Granted, if you were giving a $10 gift card as a stand-alone gift, that would probably come across as not so generous, but a $10 rattle or board book would be a lovely present to receive from a colleague. I think it's totally reasonable to tailor the size of your contribution to match your relationship with the recipient, but also, it's a group gift card, and the mom-to-be will probably assume that the final cost was split evenly among all of the givers. Even if you throw in substantially more money than anyone else, your share of the gift won't get communicated. Similarly, if the next time one of these collections happens, and the recipient is someone with whom you don't have a good relationship, or someone you really don't know, feel free to decline to contribute or to give a token sum along the lines of $5. If you decline, though, it's only fair to decline to go to the office shower as well, which may create some awkwardness, but it's still your money and your decision, and you can always schedule a dentist appointment at the same time to avoid any suspicions.
Since I can't resist the opportunity to talk about what makes a good baby gift, let me digress for a moment to say that I'm glad to hear your office is purchasing a gift card and not a designer baby blanket or a diaper cake or a giant teddy bear. I have become very self-righteous in my assertion that I know what new parents need better than they do, and I rarely look at baby registries for any reason other than to laugh (wow, I really DO sound like a curmudgeon today!). New parents don't know what they need and won't know what they need until it's 2:00 a.m. and the baby's crying and they can't snap the darn snaps on the sleeper and they say, "Oh THAT's why some sleepers have zippers." Then at least they'll have the gift card and can go online and buy the thing they actually need.