There are some UI elements in GTK+ that are touch friendly (and therefore good to use on a phone) while others are not (e.g. combo boxes).
This page will explain how to make the currently non touch friendly parts of an app more touch friendly. This page is still a work in progress. Until more is written, please take a look at these touch friendly applications as examples:
Any touch friendly UI elements should be good for apps on the Librem5.