Using GPL, you have to share your source code (or give it on demand, but the former is preferable) as soon as you release a public build. If your beta is public, you should release the source code. If you want to tidy the code before a proper release, make a private beta for testers.
You don't have anything specific to do related to the original work; it's always nice to contact the authors and show them your port, and to credit them of course.
About repositories, you could use Google Code, Github, Sourceforge... I'd say the best of them is Github, but that's mostly a personal preference.