Is the bracket of a graphics card grounded?
The bracket is certainly grounded to the case by mechanical contact when installed.

However, it is up to the card designer as to whether the bracket is directly bonded to the card's power supply ground. I would actually expect that there would be some isolation via some combination of resistor/capacitor/diode in order to avoid creating ground loops through the motherboard. But this would still provide some degree of protection from ESD if you handle the card by its bracket when not installed.