If you make a device and use it yourself, you have no obligation to share the code and documenting your changes in the code would be your only obligation. If you give the device to someone else to use, then sharing the code and displaying legal notices comes into play.
The gplv2 states -
If the modified program normally reads commands interactively when run, you must cause it, when started running for such interactive use in the most ordinary way, to print or display an announcement including an appropriate copyright notice and a notice that there is no warranty (or else, saying that you provide a warranty) and that users may redistribute the program under these conditions, and telling the user how to view a copy of this License.
So for gplv2 the firmware will at least need to contain legal notices for display. It doesn't have to be the full gpl text, you can give instructions on where to read the full license, similar to the short per file gpl header.
So if there is no UI, say the device only responded to commands recieved wirelessly, then you may be able to not include legal notices on the device.
Having said that - for this particular use, it will be a matter of can you be prevented from complying with the license? Those that want you to share the code will not care for legalities and those that care for legalities will be preventing you from complying with the gpl and the gpl terms will be the least of your concerns.