The only option to work with 265 that I can see is to download Adobe Premiere Pro. You need to also download Adobe Media Encoder too. Both programs support h.265 encoding an transcoding.
If you wish to use FCPX to edit with you can transcode the h.265 to ProRes using the Adobe Media Encoder and use those ProRes files in FCPX. Or, use Adobe Premiere Pro for editing. Adobe Premiere Pro can edit h.265 natively with no transcoding to ProRes. You can have Adobe Premiere Pro transcode h.265 to ProRes along with proxy files if you wish too.
You can download a 30 day trail of Adobe Premiere Pro/Adobe Media Encoder from Adobe.
I just did a test of the above and everything works. I was using 4k footage.

BTW... I use DivX to playback h.265 files. I have found it to be the best h.265 playback player for the Mac. Beat the VLC for h.265 video playback hands down.
http://www.divx.com/en/software/player/features
In closing... I have no idea of what kind of h.265 videos files that GH5 makes. Old-timers in here from past experience know that camera companies tend to make there own proprietary codecs. For example h.264/mp4 video files. Which for the large part you have to jump through hoops to make work on your computer. If you choose to download Adobe stuff I hope the h.265 that it makes will work with no problems.