My phone is a GX290 (not plus)
These are the step I took:
First I went back to stock Android version 9.
Update to first version of 10 (it takes 3 updates): Gigaset_GX290_10.0_V08_20200617_20200617-1948
reboot to recovery (vol-up + power)
select reboot to bootloader
some users will need to ‘sudo’ each command.
…/platform-tools/fastboot flash recovery recovery.img
…/platform-tools/fastboot flash system system.img
…/platform-tools/fastboot flash boot boot.img
…/platform-tools/fastboot reboot recovery
#wait to you see Recovery to continue
Apply update - apply from ADB
…/platform-tools/adb sideload e-0.23-r-20220512-UNOFFICIAL-GX290.zip
From the recovery menu select Factory Reset.
Format data + Format cache partition
Reboot system now.