I am pretty sure it is just a hex allen key style wrench. It is a recessed hex in the plug. They are items 34 in the diagram attached.

 

I'll get the exact size wrench you need in an hour or so and post it here - unless someone else chimes in before hand...

 

On my 72", it was a standard allen.

 

Don't know if this will help you now that I realize you have a 60D mower and I have a 62D - maybe someone can verify the wrench size for the 60D gearbox plug?

On mine it takes an 8mm hex key. Since you can usually only buy the hex keys in sets you can get a metric hex key set and you will have the correct size anyway.

You can buy these sets just about anywhere - Sears, Lowe's, hardware stores, etc.

If this is the first time removing the plug it may be a bit tight. First thing is to be sure the hex key is seated properly - use a hammer and tap it lightly a couple times to be sure. If you need more leverage you can use a box wrench like shown:

Have a rag handy and lay down below the plug hole. Sometimes a little gear oil will spill out the hole. That's means it's full and just replace the plug. If you don't get any to spill out use something to stick in the hole to see what the oil level is. It should be just below the hole.