I'd go with these for authentication and facebook oauth.
When you're unsure https://www.ruby-toolbox.com/ is a good bet to find which gems are popular and well maintained.
Another thing I do is search railscasts for an episode on what you're looking for e.g. paypal and see how he's solved it and look at the comments from others.
For paypal I've always rolled my own because the API is pretty straightforward, there may be gems but I haven't used them.
Another thing you can do is join a mailing list like railsoceania and ask the question there, you get good timely responses from a knowledgeable bunch.
Hope it helps,