I had some problems with gtags mode map at some point. I wanted to override the keybinding and use the helm-gtags key instead.
(define-key ggtags-mode-map (kbd "M-.") 'helm-gtags-dwim)
Since what you've tried to do isn't working with a global setting, it may be shadowed by a local setting or a local map. If you find that it's just a local binding, you can use local-set-key in a mode hook to override it. If it's in a mode specific map, you can use the code above as a template for your rebinding.
Update: changed the answer based on the culprit more likely being a local binding shadowing the new global setting.