I live in the U.S, so is it available to install and setup and to use it?
If it's available, what kind of GSM Modems do I need?

If your question is about the licensing, please view GPL.

This software runs on GSM network and I assume that you are using CDMA, right?

Currently CDMA devices are not supported, because they use different command set than GSM with PDU mode. With small changes to the source code, CDMA can be used to send messages, but receiving side is much more complicated. I have no plans for implementing CDMA support because I'm not able to test CDMA devices.