Prerequisites for importing from GDrive to to Rewatch:
You must have access to view and download files in the GDrive drives to be imported from.
Optionally, you must have an API token for the Rewatch API if you intend to upload to Rewatch via the API.
Keep in mind that the user performing the import will be tagged on Rewatch as the uploader for all videos being imported.
The simplest option is to import the videos from GDrive via the Rewatch web UI. It's possible to import multiple videos at once from a shared drive. See https://help.rewatch.com/en/articles/5681249-import-your-videos-and-google-meets-recordings-from-google-drive for more info on importing directly from GDrive.
For a larger number of videos, it may be preferable to download the videos from GDrive to a local computer before uploading them to Rewatch via the web UI or the API.
Options for downloading videos from GDrive:
Use the GDrive web UI to download individual video files or whole directory trees.
Use the GDrive Desktop app to mirror uploaded GDrive files onto the local computer.
Once the videos have been downloaded from GDrive, they may be uploaded to Rewatch via the UI or the API.
See https://help.rewatch.com/en/articles/5687773-uploading-videos-to-your-channel for details on uploading videos via the web UI.
See https://help.rewatch.com/en/articles/5687776-uploading-videos-using-the-rewatch-api for details on uploading videos via the Rewatch API.
The Rewatch API is currently in private beta. Please contact Rewatch to inquire about beta access.