Change in Gravitational Pull in a swinging pendulum?

You seem to be randomly grabbing at formulas without understanding the meanings of the symbols. You need to understand the principles first and understand what the symbols mean.

"Ok so I have tried finding the height using H/L = sin(37.3)"

The sine of 37.3 degrees would be the horizontal displacement divided by L. You could use cos(37.3) = H/L. That would give you H, which would be the vertical distance from the top of the pendulum to its current position.

(You have no picture, so that paragraph is based on the assumption that 37.3 degrees is measured from the vertical. If it's measured from the horizontal, then sine is correct).

"then plugging that height into Pe= mgh"

If you want to know the PE relative to the bottom, you need to use the vertical distance relative to the bottom. H is not that distance. As I said, if you use the cosine, it's the distance from the TOP.

If you subtract it from L, you'll get the distance from the bottom, which is the correct number.

"or more simply mgL(1-cos 37.3)"

That is correct (unless as I said 37.3 is measured from the horizontal, not the vertical). That's the calculation I just described.

"should my answers be negative?"

No, if B is higher than A, then the potential energy is higher.

By the way numbers in physics have units. This energy will be in Joules.