Never saw that on the 2 GB's I've owned. Only sure way to know what it's for is to throw it away. In a week or so you'll discover what it's for.
It's a "glut". You hold it upright, fill it with beer, and when you tip it to drink from it, it goes "glut, glut, glut..."
Why don't you have an emergency tiller on that GB42? They come equipped with 'em as well as the hole in the deck needed to attach them for action.
I do have an emergency tiller.
So what you found was a spare arm. There would be no need to separate from the main assembly as pictured. Glad this is over I did not sleep last night,Found it !
https://www.amazon.ca/Professional-Powered-Line-Hauler-Puller/dp/B01MQ19LVK
It is the arm of a trap puller.