There is no definitive answer to this question as it depends on a variety of factors, including the specific GPU in question and the quality of the custom resolution. That said, in general, using a custom resolution can potentially damage a GPU if the resolution is not within the safe limits for that particular GPU. It is always best to consult the manufacturer's documentation for your specific GPU to determine the safe limits for custom resolutions. If you are unsure, err on the side of caution and do not use a custom resolution that is outside of the safe limits for your GPU.