Gravitational potential energy is
(mass) x (gravity) x (height) .
When you see this formula, you should be asking yourself:
"Self ! 'Height' above WHAT ?
The answer to that question is: It doesn't matter. It can be the height
above anything, as long as you make very clear what the reference
(zero) level is, so that it doesn't change during the course of working
with the problem.
There is no particular level relative to which the gravitational potential
energy must always be measured.
In a large number of cases ... like when the problem involves something
going up and down hills, or roller coasters making loops, or soccer balls
or golf balls being launched ... the reference level for gravitational energy
is the ground, because it's THERE and it's convenient.
But if the action in the problem takes place in an office on the 80th floor
of the Aon building in Chicago, or down in a coal-mine shaft in Kentucky
where da sun don't shan, then the floor of the room you're in would be a
much wiser and more convenient level to adopt as the the zero-reference
level.
There's no law.