I just found a suggested install routine for the Gigaset GS4 using Lumo AI from Proton.
I’m not sure if I will do this, because I am new to all of this.
This was the final summary:
8. Summary
Unlock bootloader (Fastboot mode, fastboot oem unlock).
Get port image (community repo on GitHub).
Flashing by fastboot (fastboot flash ...).
Restart device and set up Ubuntu Touch.
If you encounter difficulties during a step, first look at the UBports forum for similar problems - there are usually current work-arounds from other GS4 users.
Good luck installing Ubuntu Touch on your Gigaset GS4!