For a system using a single Radeon HD 7770 graphics card AMD specifies a minimum of a 500 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 23 Amps or greater and have at least one 6-pin PCI Express supplementary power connector.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Combined Continuous Power/Current Available on the +12V Rail(s) is the most important factor.
The Gembird CCC-PSU6 500W Power supply, with its +12 Volt continuous current rating of 17 Amps and with NO 6-pin PCI Express supplementary power connector, is insufficient to power your system configuration with a single Radeon HD 7770. Judging by that PSU's specifications it is based on an old design that emphasizes power on the +5V and +3.3V rails (i.e. 56% of that PSU's total capacity). Its single +12V rail only supplies up to 41% of that PSU's total power capacity.
If you would have purchased the more modern Gembird CCC-PSU6X instead, it would have worked, because it has a PCI Express power connector and a combined +12 Volt continuous current rating of more than 28 Amps.