Despite seeming logical, because of the order things load when activating a plugin, hooking “activated_plugin” is not how to run plugin code on activation. You call register_activation_hook() directly inside of TU_ActivateGravForms::__construct() or external to your class, but within the plugin. The passed callable is the class method that will be called when your plugin is activated. Refer to the Codex entry.
Within that registered class method, to activate another plugin, call activate_plugin(). I don’t really know if activate_plugin() will work in this context since we are in the process of activating another plugin, but it’s the first thing to try. If it fails, you may need to schedule an event in the near future that activates the plugin.
BTW, when posting code in these forums, please demarcate code with backticks or use the “code” button of the editor. Otherwise the forum’s parser corrupts your code, making it difficult for others to test your code themselves. I fixed your previous code just because I can ??