You'll never be billed for programs that you download. Most software that you're able to download without paying for upfront comes with a trial period that gives you a chance to try the software before you make the purchase. Some programs will stop working after the trial period until you pay or it may become "crippled in some way so that you can still use it but it's not as convenient to use as it was during the trial. I'm not sure about Gatekeeper, but Automator comes free with Mac OS X. I just googled Gatekeeper and see that it's an app that has something to do with Voice Over IP. It has a GNU General Public License and is available at SourceForge so there's a high chance that it's a free program but if you end up using it regularly you should return to SourceForge and pay your respects with a fair donation.
So no worries, there's no bill on it way to you.