after changing the oil and the filter i would have run the bike up to temperature and then dropped the oil again, this eliminates any last bits of moisture that would have been present in the oil (as you never change all of it, some of it always remains in the engine), after filling with fresh oil go and ride it.
if the HG has gone you will notice the common symptoms of either high coolant temperatures, excessive bubbling in the radiator and/or expansion tank, excessive white steam/smoke from the exhaust etc. until you know that the HG has gone i wouldnt worry, or if you want to change it as a matter of course then change it, the gasket is not expensive (~£30 from kawasaki) and you can do it with the engine still in the frame.
i would also take the opportunity this presents to check your cam chain, and check your valve clearances as they need doing on a regular basis on these high-reving bikes.
also, as the bike has been stod for a while you do normally get a bit of cream on the inspection window, on the inside of the clutch cover and in the oil filler cap, this is all normal...however if when you dropped the oil, the whole oil was creamy then HG has probably gone.
Drain your new oil. You need to find the cause of the water getting into oil. Most likely head gasket failure.. I've read it's common.
You can reuse these.. Clean off excess flakes, coat liberally with copper spray or the loctite gasket maker resin, excellent stuff. Did this on 4 bikes never an issue and running years later..
Check the block with a super fine feeler gauge and straight edge, it mustn't go thru at all. Head as well.
Check for any nicks,even a tiny nick will leave a bulge. Block sand it flat with water and 1000-1400 grit..
Just check everything, then pour your oil back in..
Watch coolant level for first 100km.. Look for any signs of leaking water esp when hot.. Idle bike with fairings off.. Make sure yr stat /fan is also working.. Can always boil stat with key on bike off to make sure fan comes on.. It un screws from radiator..