I have been considering reading the manga, as it goes further in the story than the anime. However, the manga is currently on hiatus, begging at the end of last year, due to the author's poor-health. So, I am kind of waiting to see if the series starts back up again.
That being said, Vol 7 of Gangsta will be released in the US on July 9, 2016. This is the most current volume that has been released in Japan.
VIZ Media also just licensed the Gangsta spin-off manga, titled Cursed, with the first volume set to be released in December, 2016. If you are interested in continuing the Gangsta story, now's probably a good time to get into the manga.
Edit: Actually, great news, the author say's she is feeling better and is almost ready to begin writing the series once more! So awesome!!
You can get Vol. 1 in print at a couple places:Amazon: (http://www.amazon.com/Gangsta-Vol-1-Kohske/dp/1421560771/ref=sr_1_1?ie=UTF8&qid=1460698212&sr=8-1&keywords=gangsta+vol+1) Barnes and Noble: (http://www.barnesandnoble.com/s/gangsta+vol+1?_requestid=1306389 VIZ Media: (http://www.viz.com/read/manga/gangsta-vol-1/10858)