PROBLEM: While properly installed and powered the GPU in question will show the grey boot screen with the Apple logo, the progress bar will begin to fill and the computer will reboot. The Mac Pro will be stuck in a boot loop and this repeats indefinitely.
SOLUTION: This issue occurs when a driver has not been installed or the incorrect driver has been installed. The Maxwell and Pascal series GPUs do not have any driver support built into OS X and are reliant on Nvidia web drivers. Links to all of the currently released Nvidia web drivers can be found here, listed by OS X version and build number.
Ideally the driver should be installed before installing your new GPU while using your current GPU. If that is not possible refer here for an alternate method.