Let's go look at what the rules actually say:
Physical Traits
These traits determine the laws of physics and nature on the plane, including how gravity and time function.
From the "types of planes" section:
Demiplanes
This catch-all category covers all extradimensional spaces that function like planes but have measurable size and limited access. Other kinds of planes are theoretically infinite in size, but a demiplane might be only a few hundred feet across.
From the description of Portable Hole:
When spread upon any surface, it causes an extradimensional space 10 feet deep to come into being. This hole can be picked up from inside or out by simply taking hold of the edges of the cloth and folding it up. Either way, the entrance disappears, but anything inside the hole remains.
So in summary:
1) Planes have their own traits, including gravity.
2) Extradimensional spaces are demiplanes, a type of plane
3) Portable holes are extradimensional spaces.
4) Therefore, Portable Holes have their own planar traits, including their own gravity.
This is directly from the RAW, requiring basically no extrapolation (all of these things are fully defined and described in the rules I quoted above).
So regardless of how they're oriented, the gravity inside the extradimensional space is going to remain constant. I guess the creator of a portable hole could decide to give it different planar traits than a "standard" one has, but I'm not sure how that would work.