If I remember correctly you do need to buy the 14 pin kit. When the "G" series machines came out and the 7 pin connector was introduced it was designed to to talk to the computer on the more advanced attachments such as a stump grinder, snow blower or asphalt planer. Years ago I had to wire a Bobcat snow blower with a seven pin to a Case skid steer. The only way I could do this is to build a box with the needed switches to control each function and eliminate the stock 7 pin wiring. The solution was kind of crude but it did work.
Check out the following link, it is from Quickattach.com, I found the stump grinder and they show a control handle that plugs into the valve body on the grinder. If yours looks the same you might be able to this and might save you money.
Good luck.
https://parts.quickattach.com/collections/quick-stump-away™-stump-grinder-900550-900551