In my emacs (24.5.1) I have both and I'm not sure which is what. ggtags was installed through list-packages from melpa.
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IIRC gtags-mode ships as part of GNU Global and gives a basic set of commands and key bindings for using Global. ggtags does much the same thing but is a much more comprehensive package with way more features. I initially found some of the ggtags keybindings to be a little confusing (especially after doing a find tag operation) but in the long run it's probably going to be a better solution than plain gtags-mode.
