This. LDS.org is the cross-platform, legacy operating system supporting version of Gospel Library (and LDS Tools) that you are looking for.The web resources are there for all platforms.
Gospel Library itself is, first and foremost, a mobile application. That Windows 8.1+ has the app at all is a benefit provided to Windows users on modern systems because of Microsoft's universal app strategy. That Windows has an app at all is nothing short of a miracle, frankly. All of our team are volunteers. As has been pointed out, none of us is paid. Heck, just getting the permissions necessary to take on Gospel Library as a volunteer-driven project was a miracle.
The Church may have the funds, but they have also been very clear that the only operating systems they have any intention to support monetarily are iOS and Android.