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Modal may not work because of Gantry also. The modal relies on the Bootstrap 2.3.x CSS supplied by Joomla, which is replaced in gantry. The module display method, however, should not be affected by anything, and page ribbon brings its own CSS - so it should display anywhere.
If you wanted something to act like a modal, you could give the ribbon a custom CSS to make it appear any way you want. Likewise, the HTML can be overridden to match your template.