It appears the gnome-search-tool is not available in Bionic. On https://launchpad.net/ubuntu/bionic/amd64/gnome-search-tool/3.6.0-2 it says status Deleted.
I've downloaded the deb from Wily Proposed repo and installed that on Bionic and it works just fine.
So why has it been removed? And what are good alternatives, specially if you want a GUI to search for strings (or regex?) inside file content not just file names?