So Einstein claims that time moves slower when gravitational pull is stronger.

A test with atom clock, positioned on different altitudes seems to prove his theory.

But

What it atoms just move slower with less gravitational pull? That doesn't mean time in itself is slower, just that our mechanism of measuring time is faulty, or dependent on gravity.

I'm sure I'm not the first person who made this assumption, so do any of you know where I can find more information on this subject?