Judging by your questions you may not be experienced enough to go exploring in a transmission.
You probably need a mechanic to assist you.
Yes there is a gasket under the cover.
It would be helpful to look at Kubota's parts book like this one;
http://www.kubota.ca/en/PartsCatalog.aspx?book=BKIDK1082
so you can see what is in there.
Manual would be good to have so you know how things are adjusted.
This will not likely be a simple fix.
In the second picture, 20 and 50 are the shift forks and it most likely one of these is bent/broken/unhooked from it's gear set.
Good Luck.