I want to install Bleachbit system cleaner, but the system does not allow installing even from AUR.
Can anyone suggest a solution to this problem?
Maybe someone can advise me to substitute with something that is equally effective for KaOS?
demm already linked the necessary documentation you can follow to build a PKGBUILD for your KaOS installation. As a starting point you could consider the PKGBUILD from AUR. If I am not wrong, the python-gobject dependency is provided by the python3-gobject3 package from main.