Why doesn't Gnome Search find your files?
Well, by default Gnome only indexes files which reside directly in your home directory or in the folders Desktop, Documents, Downloads, Music, Pictures and Videos.
Is there some way to search in other folders?
Yes, there is.
To add a folder to the index open
gnome-control-center, open the "Search" menu, click on the little gearwheel symbol and switch to the tab "Other". Now you can add a folder of your choice by clicking on the plus symbol, unfortunately this works only for folders located on the same drive as your home folder.
How to index files located on a different drive than your home folder?
For that you need to bind the directory you want indexed to a folder located on home's drive.
For example use:
sudo mount --bind /some/directory ./searchdirectory
To make it permanent you would need to write a new entry to "/etc/fstab".
For example like this:
/some/directory /home/youruser/searchdirectory none bind 0 0