A truly uniform gravitational field is inertial, yes. But those don't really exist (we say, for instance, that on Earth the acceleration due to gravity is -9.8 m/s^(2) but there is a slight height dependence on it). So, things in free fall (without air resistance- including orbits) we will say they are in "locally inertial" frames. But even in the ISS, there will be slight tidal forces acting on you- aka, the side closer to Earth will have ever so slightly more gravity than the side further away.