CM13 aka cyanogen mod 13 for HTC One M9 can be downloaded in form of nightlies. This comes without any gapps but you can install them if you cannot live without them. You can download them from opengapps.org and make sure you flash the package immediately after install the ROM – before booting up.
Keep in mind that since it is a nightly build, there might be some annoying and non-annoying bugs that may or may not be ironed out in upcoming builds.
CM13 for HTC M9 brings android 6 marshmallow which is the latest version of android operating system. HTC might take a while to bring official android 6 to HTC M9 but thanks to XDA, users of it will be able to get taste of it for quite a lot of months before it is officially released.
All the source code for CM13 for HTC M9 is available at the github repository.