Just a few thoughts.
6 Gallons is a lot of gas in there.
I am sure there is still gas in the oil.
A normal oil change does not drain the transmission or the oil galleries, but the engine and trans do share the same oil.
Now that you ran it, do another oil change, this time also pull the bottom bolt out of the middle gear cover to drain the trans. Have a new copper crush washer ready for that bottom bolt.
The gas could have washed all of the oil out of the clutch pac, but I would think that would make it grabby, not slip.
Since gas and oil are pretty closely related, I would be surprised if it actually damaged the clutch or any seals.
Try running it for a little while, in gear and clutch pulled in, as if you were at a stop light, this may help flush fresh oil into the clutch plates and get the gas out from between them.