Codec for raw images from Canon G5X Mark II

Which system version is installed on your Mac? The RAW format of Canon PowerShot G5 X Mark II is already supported on OS 13, iPadOS 13, and macOS Catalina.

To get the RAW support in Photos and Preview upgrade your Mac to Catalina, if your Mac is compatible with this system, see: Digital camera RAW formats supported by iOS 13, iPadOS 13, and macOS Catalina - Apple Support

You can tell which cameras your system has codecs for by holding down the Option key and selecting System Information under and  menu. In the Info window click on Raw Support in the sidebar and you;'ll bet a list of all camera whose raw files are supported:

Lightroom can open and handle G5X mark II .CR3 files. As of version 11.1, Graphic Converter can as well.

I am running the latest version of Mojave, and after I installed the last security patch (which did not change the version #), I noticed that the system supported the CR3 files from my G5X Mark II

Adobe Photoshop Elements and GraphicConverter are two I know that can edit RAW files. Don't know if they can handle your camera's raw format but you can run GraphicConverter in demo mode to find out.