I checked a bit more and it seems that GL to SD functions fine. When I mount real racing 3 a folder "com.ea.games.r3_row" of 1.39 Gb suddenly "appear" in Android/data. This folder is a perfect "copy" of what's in the ExternalSD. If I unmount the game the folder disappear. But before and after my disk space remains identical.
In other words GL to DS mount an image, making android believe a folder is there. But it is here only virtually. The physical folder is in the External SD. It's the same idea as Daemon Tools on PC. You mount an image .img .iso .nrg in a virtual DVD. You can see the content of a DVD in windows eplorer but you know it is not really there. Daemon and GL to SD force your OS to believe that some files are where they are not in order to make a program start.
Other thing that may sound weird : some big programs appear in GL to SD with 0 kb or a very small size. GL to SD moves only what's inside data or obb folders. If a game have nothing in there then no space can be freed :/ Except by moving the application to SD card if your phone does that (HTC, etc). Samsung doesn't. ... which sucks big time.
I tried dozens of "move to sd" apps from om the store on my Samsung S3 but none work on this phone. GL to SD manage to do something really nice already on rooted device. If someone know an app that moves games/app on External SD for devices that does not allow that please let me know ! Thanks