I have a shower stall that is leaking through the ceiling into the
kitchen below. I have poured water from a one gallon water can directly
into the shower stall drain pipe and the leak occurs when I do that.
Based on the results of that test I don't believe the leak is from any
leaky gasket at the shower pan drain or bad grout around the edge but a
leak in the drain pipe it self and most likely in the drain trap section.

Does this sound like a reasonable assumption?

My next step would be to remove a piece of the ceiling below the shower
in the kitchen. Depending what I find, I will either attempt a fix
myself or call a plumber. I have replaced traps in a kitchen sink but
don't know if that's doable in shower stall plumbing.

Any suggestions would be greatly appreciated.