So I've added the 3-prong plug to my garbage disposal, and now I'm on to the GFCI outlet installation. The breaker was not GFCI protected, nor is there a GFCI outlet on the loop. I've gotten to the point where I need to know which is my line, and which is my load given this was a switched box.
Given the following was what I found, I'm pretty certain the LINE is coming off the left and the LOAD heads off to the right. Updated the drawing to reflect the actual situation.
Am I right or should I try each pair on the GFCI as the line on their own before wiring it up as I believe it should?