[Floridian]

From past experience CAN on buses it won't be just a single message, it is a group of different messages that will allow something to happen. 

In your case you should think about gear selection (think about it, if the transmission is already in first gear why change it to first gear? or if it is in reverse you can't just change it to first if the revs are min. X, or > X) it all depends on the computer for you to be able the trans to do what you want. You'll need the original ECU and everything connected, filter those ECU's and then see what are they interchanging at the time whatever device you need to control.

My experience comes from using BMW iDrive controls on my legacy coach to change gears (I detect the CAN bus messages when any of the iDrive devices do something and then act on my old newel like it used to with a button).

I used a CAN bus to USB tool that helped a LOT to decode some messages but some other ones were really obscure (for example, BMW will physically move the gear stick if you send a specific message or if a certain condition is met).

If there's anything I can help with let me know but unless this is some very basic CAN buses and ECU's it will take more than just those two talking to each other.