Make your own out of a weed sprayer. I removed the sprayer handle, inserted a barb-pipe thread adapter into the end of the hose, then a ball valve, then another pipe thread-barb adapter, and then several feet of clear vinyl tubing.
Close the ball valve, fill it with the prescribed amount of oil, pump it up, insert the end of the tubing into the filler plug, open the ball valve, and then go do something else. Come back once in awhile to pump it up again until your transmission or differential is full. If you put exactly the right amount of oil in to begin with, you'll just have a few drips from the filler plug.
I made two, one for GL-4 and one for GL-5. They cost about $15 each to make, maybe $20. A one gallon sprayer is typically about $10. Mityvac and Motive Products and probably others make essentially the same thing but you pay a lot more.