Go to .gs file/project(Tools>Script editor..) which you want to use as a library.
Save file with the version at File>>Manager versions.
Copy project key from this menu location File>>Project properties>>Project key
Go to your second .gs file/project and add previous project key as a library at menul Resources>>Libraries…
“Add a library” -> click on Add button -> click on Save
Add identifier for this library once library is added with Resources>>Libraries… menu.
Now you can use functions from previous project/library with this identifier inside your local function.