Each android device has a "build.prop" which lists various properties about the device such as screen resolutions, sensors that are present, and so on. Each app has certain requirements as to what devices it supports, based on the build.prop values. And yes, if you are using the app on your phone, it completely hides "incompatible" apps.
Normally this would block apps with an incompatible OS version, or missing hardware, something like that. Sometimes, especially with older apps, it might mean they have a problem for whatever reason with certain hardware so they haven't enabled it. It might not like the screen's aspect ratio. It might be a complete oversight. You can try contacting the apps's support and asking them if they plan to make it available for the G6.
If you have some way to get the apk off your wife's phone and move it to yours, you can sideload it. Then you'll find out if it really is incompatible!