Why has gnome-search-tool gone?
Solution 1:
It was removed for being unmaintained, but you can use mate-search-tool instead which is available from the repository and is exactly the same thing.
sudo apt install mate-utils
mate-search-tool
As an alternative you can use searchmonkey which performs the same task.
I've been using it for awhile now and I like it a lot better because it will show you a preview of how your search phrase is used in each file allowing you to quickly scroll though the resulting files without having to manually open each one up.
As a side note, if you're searching the entire filesystem, searchmonkey gets lost in /proc and takes forever. To solve this I use the find command with -xdev to search only real files in the current filesystem like so:
find / -xdev -type f -print0 | xargs -r0 grep -Hi 'your search string here'