I track down the main issue as follow :
GDE includes functions at the top of gviewer.php:
Now, NGG has a couple of special AJAX calls (yes, will be rewritten in the future) to show a image (edit thumbnail) or upload a file, where it reload the WordPress functions via a second call (ngg-config.php), in this case all filter & action hooks will be executed again, but due to the fact that the "functions.php" are not yet loaded, PHP throw a error.
A simple,intermediate solution is to move gde_conflict_check() and gde_t() to gviewer.php, then it should work fine.
I need to recheck if it's not better to use here the action hook "plugins_loaded" , I think here is the main problem at my side