just take the rear cover off to start with. 4 bolts in the outside edges. Clean it up. Then go from there.
One thing worth mentioning and I have seen this on original unmolested transmissions with the GR series (2100, 2110, 2120 all generations) is when the case halves are assembled, they use liquid gasket. Integral to the case halves is the case VENT channel, runs from bottom to top, and everywhere between. The vent channel is very close to the liquid gasket area and sometimes they'd get too much liquid gasket on it, which would plug the vent channel and cause the trans to vent fluid.
If you want to play with a POOR design of a transmission, play with a GR series. Absolutely stupid! If you end up having to split it, prepare yourself. You will need a 10mm socket and wrench, an inch pound torque wrench, a piece of 3/32 (IIRC) starter rope about 6 foot long, and tons and tons of patience. When you reassemble, make sure the case halves (which are a poor casting, you will see what I mean) are squeaky clean, lay the rope into the vent channels, apply liquid gasket. Assemble the halves and snug them together, IIRC there are 43 bolts-and 43 nuts on them. May not be that many but it will feel like 50 or so. Before the liquid gasket sets up, pull the rope out. That keeps the channel clean. Then go ahead and torque all of those bolts. If you don't get frustrated with the transmission at that point, you are inhuman and I will bow to your patience.
If you let the gasket set up before pulling the rope out you will not be able to pull it out and you get to take it back apart again, clean all the gasket off and try again. Speaking from experience!!
Another leak point I ran into was the pto clutch housing. The clutch is a multi-plate setup which requires a special tool to get it properly into it's dumb design of a case. If you don't have the tool, you can rely on luck. If you don't get it quite right, you'll crack the clutch case and/or transmission case (guaranteed). I have done it. You don't see any cracking right off, but it will leak fluid, slowly. After running 4-5 hours at full throttle it had a couple of dime-sized puddles under it. Customer wasn't happy about that, being that he spent like 10 grand on the pile of steaming feces. So out it came. Over and over trying to find the leaks. We ended up eating the cost of the transmission, at that time about $3500, replaced it as a unit and solved his problem; albeit temporarily. That was only one. There were several. The funny is that the customer said keep that piece of junk transmission and do whatever with it, which I found the leaks and wasn't able to really repair them without $$$. My boss moved the business to a new building which had a bunch of trees that we had to cut and pile up. Well we touched off the one pile and I decided that the best use of a GR transmission was to put it on the pile and watch it BURN. That was the only satisfaction I ever got out of working on GR series mowers in the 28 1/2 years I worked at a kubota dealer.
