Code is open source if the code is available (online) under some open source license. This is true for all approaches on OpenSlam.org . If you manage to compile and run it tells you something about the ease of use of the approach and your skills in compiling other people´s software, but really doesn´t have anything to do with "proving" the approach is open source or not.
That being said, some compilation instructions can be found here, but I´m not sure if those work out of the box. You might have to read up on how Makefiles work if you want to compile gmapping yourself. Note that this question really goes beyond the scope of this site though, because ROS Answers is about ROS related answers (and compiling gmapping standalone has nothing to do with ROS).
You´ll find some SLAM approaches for ROS that are not uploaded to OpenSlam.org , but the latter definitely is the largest collection of SLAM approaches on the net I think.