Hi Stephan,
Enabling secure boot AFTER an OS is installed is not a good idea. Secure Boot is supposed to provide a chain of trust from the BIOS to the OS hand-off, but the OS needs to be aware of this chain of trust when it is initially installed. Otherwise it assumes that your system does not support secure boot, and it installs the OS without support for Secure Boot.
Additionally, your off board video card will cause problems for you. Unless you updated the firmware on the video card to have a Windows 8 signed Option Rom, enabling secure boot will cause your video card option rom to not be trusted, and therefore not executed. If this is the case, you could stop seeing video displayed from your off board video card, in which case, the system MIGHT switch back to using the onboard video device. This switching depends on how the BIOS settings are configured by Gigabyte. If you are not seeing video output on your offboard card, you should try connecting your monitor to the mother boards video connectors and attempting to boot the system.
Also, as part of Secure Boot, the bios has something called "Fast Boot", which is supposed to decrease the amount of time required for the BIOS to execute. If you enabled Secure Boot, then you will have to hammer the Del key to get into setup.
The easiest way to return your system to a working state will be to clear CMOS. There will be instructions in your motherboard Manuel on how to do this. Once you do, go back into the BIOS setup utility and make sure that Secure Boot is not enabled.
If you are still having problems after clearing CMOS, try this: disconnect all bootable devices. Disconnect all devices connected to your SATA ports, all your USB mass storage devices, etc. Then power on the system and hammer on the DEL key. It may take a couple of attempts, but eventually, you should be able to make it back into setup.
Good luck to you. I hope you get your system back into a working state.
