I would guess so, but i want to make sure.

Is the bracket of a graphics card grounded to the ground-plane of that card?

The metal bracket is the thing with the DVI, HDMI, DisplayPort etc. connections.

Edit:

After measuring with a multimeter, i suspect that brackets (of most graphics cards) are grounded to the graphics card's ground-plane.

I measured 0 ohm between the traces around the motherboard screw holes and the bracket of a (old) 75W graphics card. (All power from the PCIe x16 slot. No power cable needed. So, graphics card's ground-plane == motherboard's ground-plane.) The motherboard was outside the case, on a wooden table.

On some newer graphics card, a screw (which fastens the bracket to the board) is clearly visible on the back side, suggesting direct contact between the bracket and the graphics card's ground. (This construction also counteracts sagging. A little bit.)

Although a graphics card's bracket probably is grounded to the card's ground, it might be possible there is some sort of non-conducting coating on the bracket. Then you'll have to touch the screws around the DVI port, to reach equipotential.