I have built my first GTK4 app for the librem5. It will not run on byzantium, because this is too old.
But I have tested it on my librem5 with mobian/trixie and on a kvm with pureos/crimson.
The description of the app is:
cashbox - memorise cost and calculate price of articles
cashbox memorises the cost of articles and calculates the total price and
change. It is intended for small clubs on a celebration, where members are
not experienced in memorizing prices and doing mental arithmetic.
The app is available at Debian / cashbox · GitLab
There you will also find the steps to install it or to build it yourself from source.
I hope someone of the librem5 community will test it and give me feedback.