TL;DR* Here's what the license entails:
1. Anyone can copy, modify and distribute this software.
2. You have to include the license and copyright notice with each and every distribution.
3. You can use this software privately.
4. You can use this software for commercial purposes.
5. If you dare build your business solely from this code, you risk open-sourcing the whole code base.
6. If you modify it, you have to indicate changes made to the code.
7. Any modifications of this code base MUST be distributed with the same license, GPLv3.
8. This software is provided without warranty.
9. The software author or license can not be held liable for any damages inflicted by the software.
More information on about the LICENSE can be found here
You shouldn't be forced to open the coprocessor firmware.
As i said the test is if someone could modify the firmware, and then install the modified version in a device that has your coprocessor and custom firmware for it. if YES, you aren't violating the anti-tivo.
Since you aren't intending to distribute your coprocessor firmware anyway, that sidesteps the entire issue. Unless you intend to distribute the modified open source firmware to others, the GPL is a complete non issue. Your modification is PRIVATE. You would only have to give someone the source of the gpl portion if you sold them your 3d printer. which would also give them your closed source additional firmware. Printing stuff with the printer doesn't count as distributing the firmware unless it is AGPL, and it almost certainly is not.
Happy to explain things.