Unless the engine has been dismantled at some time, it is unusual for a Honda to leak oil through a gasket. I suggest your next step should be to read through these two threads:

https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=49392&page=2

https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=52741#Post52741

At the moment I'm having some difficulty finding the joint face, where the gasket is, in your picture (though I see the oil of course). Is it the gasket inside the green oval in this picture?
[Linked Image]

(That picture is of a GXV120 not a GXV140, but the crankcase cover is the same part, and the cylinder/crankcase is the same except for the bore diameter.)

If it is the crankcase cover gasket that is leaking, we still have the question of why. Did you check for oil in the rubber breather hose? If the problem is a stuck PCV valve, you cannot cure it by increasing the clamping force on a gasket: all that would do is cause it to leak through another gasket. On the other hand if the engine just has a loose crankcase cover, and the previous over-filling with oil had nothing to do with the problem, tightening the cover bolts would probably be a complete cure.

By the way, I notice you do not have a workshop manual for that engine. Here is a free download of the "short-form" version of the manual:
http://www.honda-engines-eu.com/documents/10912/31969/1551/b10d7687-f6e2-40e0-9faa-ef36e383cc9b