Since Guilds of WoW is a Progressive Web Application you can install application via browser on Windows and Android.
If you don't see the installation button please use Chrome or Edge or check back later...
Unfortunately iOS doesn't support PWA installation yet. Fingers crossed for further developments on Apple side but you can install the application by using the 'Add to Home Screen' functionality.
1. First click the Share button down below.
2. Then navigate to 'Add to Home Screen' button by scrolling down.
3. Later simply click the 'Add' button. Voila!