This is a two-pronged false friend. Free doesn’t mean “libre” but sometimes it does, and it doesn’t mean “gratis” but sometimes it does.
A much easier way of saying the same thing is that “free” has two translations in Spanish, related to its two implied meanings in English – free as in “open or liberated” and free as in “you don’t have to pay”.
When someone is let out of prison, or when you leave a job you hate, you might say “I’m free” !!!! “¡Estoy libre!” A teenager challenges his/her parents with “I’m free to do as I please” (“Soy libre para hacer lo que me da la gana”). Notice the use of “ser” and “estar” here (I’m in a state of freedom – estar; I’m a free person – ser. You can watch my video which explains this difference at: http://www.janecronin.eu/index.php?option=com_content&view=article&id=300 )
Back to the point, “libre” means “open” as well, that’s why when you see a sign that says “buffet libre” it doesn’t mean you don’t have to pay for it, but it is open to any paying customer to partake.
“Free” meaning “you don’t have to pay” in Spanish is “gratis” or “gratuito”. If an advertisement for an event says “entrada gratis” then your wallet is safe. Interesting “gratuitous” does not mean this in English – yet another False Friend.
Whilst on the subject of “gratis” most of the things on my website (such as the video link above) are “gratis”. A sign of the times is that I’ve now introduced those googly link things that everybody now has. They will help to keep the site free if people follow them up. Internet is a wonderful “free” thing!