Remember: I don't own a GS. So take this with a grain of salt........

I would say taking the exhaust off is mandatory (don't snap any studs though). If you can access the bolts on the left side of the pan (if you were sitting on the bike), then the front / rear/ right sides shouldn't be a problem.

But wait for somebody else to confirm.

The oil pan is below and between the sides of the frame. You can take it off if you remove the muffler.
I would not turn the motor upside down. The idea is to see what's fallen into the oil pan - if anything. Upside down would let what's there fall back into the engine. You want to examine it for crap. If you were servicing the engine, then you would need it upside down. To check for crap, take it off without disturbing things.

Ok, I will do that when the gasket arrives. For now, I just drained the oil after putting some seafoam to see if it would loosen some stuff up if it was hanging around in the sump filter or oil pump. I didn't run the bike, however, so I don't know how well it would have worked. I just drained the oil and took off the filter. The filter had no oil in it... More precisely, it had had oil ran through it and was saturated, but there was no oil actually in the filter like there usually is. Could this mean that air was trapped inside and was not allowing oil to be pumped into it, thus causing the problem; or, what I hope is not the case, that the pump just wasn't pumping oil into the filter?

I'm hoping for the prior as I do NOT want to dismantle the clutch to get to the pump. Here's to hoping. Hopefully the sensor and gasket will be in tomorrow and I can pick up a new filter at napa. Crossing my fingers.

Edit: no metal bits were stuck to the plug, so that's a plus.

Air will push right through the filter.  You have a blockage in the short distance from the pump to the filter, or no oil pressure. 

http://www.familyjones.org/paul/Engine%20lubrication%20chart.pdf

Think to remember: Many of the metal bits could be aluminum. all of Junior's metal crap as aluminum from the inside of the case - no steel. Crap can also be non-metalic.

Ahh! Stop scaring me! Still waiting on the gasket and pressure switch. Should have been here by now. I'm going to pick up the new filter and take off the oil pan to check when I get back in town. I'll just leave it off for when the new gasket gets here. I assume the old gasket is unusable once you take the pan off?

Edit: Will the screen be removable with the engine still in the frame as well in case it needs to be cleaned?

Screen is screwed on tight. You might be able to get it out with the correct JIS screwdriver. I had to use an impact wrench.

If there is stuff on it, you can probably clean it out with it there. However, if there is stuff, try to figure out what it is.

Hopefully just a sensor or some filth came lose. Might be the change in oil. Might be a problem with the new oil filter. These things happened shortly before you had the problem.

Could the oil filter be off place? Maybe whoever out the oil filter in lost the spring on the oil filter cover? Knocking bike over made filter get out of position without the spring?

I have an impact gun, but without a proper fork stand or jack, I don't think it would fit. But, yeah, I don't think it will need to be taken off either.

The oil filter seemed snug in there and the spring was still in the cover. It all looked kosher except for there being no oil in the filter besides the saturation from previous proper working conditions.

I'm headed out to the garage to dump the cheap walmart 10w40 and the new filter in and see if it works. If not, I'm praying it's a clogged sump filter or a bad sensor... but the bad sensor is looking less and less likely a culprit now that I've seen the old filter.

Ok, the little bastard lives!!!

I just changed the oil... after all that, that's all I had to do. My youngest daughter is now my good luck charm, and I will drag her into that garage every time I touch one of my bikes.

Thanks to all that walked me through this issue and didn't let me give up. Maybe the filter was clogged and I didn't notice. Maybe there was an air bubble in there that prevented pressure. Maybe the seafoam worked. Who knows? So, Juan, if you're still following this, change your oil and filter

On another note, anyone in need of an oil pan gasket and pressure switch? Lol.