Oracle Global Temp table - data is truncated but the table remains in the DB ! is this an expected behavior or a configuration?
As the comments indicate Oracle Global temporary tables work differently than some might expect.  They are intended to be permanent structures.  The settings allow for data to be retained when a session is closed or deleted.

The key reason to use them is that each user can only access data for that user.

Your developers can all use the same table and insert/update/delete without interfering with other users.

There is no need to drop a GTT after using it.