Get on HP's support forums and ask what the size of the screw is before anything (99.99% chance it's a metric screw). HP's support forums are full of knoweldageble people including paid HP tech support people. I've used them for years when asking for help rebuilding laptops and missing screws and have never been let down. And no, don't run that GPU without full screw lockdown as another said here. GPU chips like CPUs have irregular hot spots and if your missing screw for the heat sink clamps down to said hot spot, you are risking burning up the card under load.