In Lightroom terms, what is the "gallery" you talk about?
A single Lightroom catalogue can manage many thousands of images. Remember, it manages the photos but they aren't physically part of the catalogue or inside it - the photos remain where they are in your folders. I work with one that controls well over 600,000 photos.
I have a 700k catalog and seriously it is slow and not too useful except as a catalog. However it is essential to have a catalog that references all images.
The first instruction is to create a new Catalog and "Add" the files to it... this then references the files in their existing locations/drives.
I would import sections at a time as it will be a long process and it means you can do it a bit at a time..... before you start on each drive ensure you have a Top Level folder on each drive so that any future moves of files/folders is easy....
When you Add the files/folders I would recommend building Standard Previews otherwise each time you view a file in Loupe View a new preview will be rendered anyway.... I would also recommend rendering Smart Previews, these enable work with a file even when it is offline which will be frequently if you have many drives.
I would also suggest you look at keeping the Catalog and Previews on an external drive that is fast and dedicated to the catalog.
Now I also have a working catalog with current (up to 200k) files as this runs much faster. This I import into the large catalog at intervals....
Hope that helps