Different countries and airlines have restrictions on mobile devices over certain sizes, or with batteries over a certain capacity being able to be taken on flights in hand-baggage.
The UK government details for air restrictions can be found HERE. You should check with your government and airline before flying with a Gemini on your person or in your hand-luggage.
The Gemini is 17.14cm (length), by 7.93cm (width), and 1.52cm (height) and contains a 4,220mAh battery.
The Gemini includes a dedicated Flight Mode key available by pressing Fn + F.