I'm aware this is an old thread, but I had a similar issue with my 2017 GC1705, and I don't see "my" solution listed here, so here's the story:
I was blowing snow with my rear-mounted snowblower, and after I had pulled forward (about 50 feet) to start the next pass, noticed a stream of oil.
I shut down, found the hole where the sight glass should be, back-tracked and found the glass. Called the dealer, he had me pull the fill cap and examine the vent in it. The vent had glazed over with ice, which resulted in enough pressure in the housing to blow out the sight glass. He had me pull one of the bolts next to the sight glass, and get a fender-washer just large enough to catch the edge of the sight glass, it is now locked in place and can't possibly blow out again.
[ On my own advice, I reamed out the vent hole a tad larger, and added a second vent hole on the other side of it. ]
-I only lost approximately a half-gallon of hydraulic oil, so there was no damage to the tranny.
There was no loctite or other sealant on the sight glass, it is just a pressure fit.