Stripping it down to the basics;
The powered people in The Gifted are mutants which spontaneously develop powers around puberty as the next phase in human evolution.
The powered people in Inhumans are inhumans which can have their powers activated by a process (exposure to a substance called terrigen.) They are the product of experiments by a race of aliens (the Kree) during Earth's prehistory.
Conceptually (from what I've seen so far) the shows are very different. The Gifted is about a family dealing with the pressures of having mutant children in a world that hates and fears them. Inhumans is about a nation of inhumans living on the moon, royal intrigue, a coup and (I assume) their first dealings with a humanity that has just discovered it has had inhumans living among them for centuries, some in secret and some in ignorance.