An external mic can definitely be used with our gimbals. The main concern about using an external mic with our gimbals is the lightning port or audio jack sits up against the tilt motor in our gimbals. However, you can definitely mount your smartphone with enough space between the port and the tilt motor to allow for a lighting port and external mic connection. Pease see the attached image below. If you are able to use a right angled connection adapter, that would allow for a better set up.
In addition, Jaymo from our office has created a "smartphone audio solutions" video to give you a visual with the types of external mic set ups you can use. This video does demonstrate the set up with our RAGE-S, but it can also apply to our smartphone gimbals. There are quite a few ways you can use an external mic so feel free to play around with it to find what will work for you.