Yes. You'll lose quite a bit if the bike is upright. If you can lay it down or lean it way to the right it can be done, but not very practical. I've done it in the field. Needs to be leaned about this much:
Let it sit there to drain into the crankcase for as long as possible before cracking the cover and there won't be much mess. Frankly if you're at home I find it's a bunch easier to just drain the oil and work on it upright or even leaned slightly left. You're bound to get some doodoo in there if the gasket doesn't come off in one piece.
Break the sprocket retaining bolts loose with the bike in gear, the rear wheel on the ground and the chain still hooked up if your sprocket is that type. If they're stubborn to remove that will keep the countershaft from turning as you try to unscrew them.
RE: the gasket. Loosen all the bolts a few threads and bounce a rubber mallet lightly on the cover until everything rings hollow. Then back the bolts off some more until you get just enough daylight to tell that the gasket is loose all the way around. If not, bounce it with your mallet, eyeball it again. Usually you can get it off in one piece and reuse it if you never allow it to tear. (The same also applies to the valve cover and oil cover o-rings. If the covers are stubborn, don't force them. Loosen the bolts slightly and bounce them a few whacks and you'll probably spare the o-rings).
When you reinstall it or replace it put a barely visible film of RTV on the gasket surfaces of the COVER only. The gasket will be more likely to come off with the cover in one pice next time and less likely to tear. If you can, do the mallet drill anytime you remove the cover thereafter and you may get a few reuses out of the gasket.