Have someone else to help you, if you watch the gear selection mechanism (or perhaps record it) - you should be able to shift through the box by simply moving the rear wheel back / forth (the gears do not line up in the box, hence why you can't shift straight through from 1st to 5th, and also why you sometimes won't be able to change gears at a complete standstill).
You should see the selector mechanism return to centre after each change. Resist the urge to put your fingers in or near the change mechanism or the clutch basket, it will all spin.
At the very least, if taking to a garage isn't an option, if the mechanism now seems to work fine, and gears work, and the clutch appears to work, and we're thinking water in oil contamination, then button it back up, put some proper M/C 10w/40 (or other oil if specced for the DL250) in it with a new filter, run it through, and possibly consider changing the oil out again earlier than specified.
If you have any doubts about it (because if the gears get stuck again, you may get stranded!), I would get a shop to look at it.
Also: Never ever ever direct a pressure washer at any external seals on a bike (well, really any vehicle). They are designed primarily to stop the oil coming out, and low pressure water and crap getting in, so don't do a good job against 1000+psi water trying to get in. I use a pressure washer on my bike only for the final rinse from a distance. Front sprocket cover crap tends to be easier to brush off dry too ;-)
