My limited experience is that it is very rare for an IMG to be in the US military as a physician. The only one I know was rejected almost half a dozen times because he was an IMG before finally getting accepted. And that was only after being board certified from a US residency.
I have never seen anyone join the Navy directly as a GMO/flight surgeon/UMO.
I did see a couple IMGs interview for residency spots once (didn't actually get selected) but it was at a different time and I don't know how hard up the military is right now.