Sign in to the Google Account that owns the library and go to takeout.google.com. On Select data to include, deselect everything, then tick Google Photos and pick which albums to include. On the next step choose the delivery method, Send download link via email or add the files to Drive, Dropbox, OneDrive or Box, choose Export once or Export every 2 months for 1 year, then choose the file type, .zip or .tgz, and the maximum file size, which splits the output into several files when the library is bigger than that. Google emails the link when the export is built, which takes from a few minutes to a few days. The link expires after about a week and allows five downloads.
What it holds
Photos and videos with their capture dates, locations, descriptions, albums and edited copies.
Find the size
Google Photos itself prints no total. Storage is pooled with Drive and Gmail out of one allowance, 15 GB on a free account, and the breakdown sits at one.google.com/storage/management, so read the Photos figure there rather than the account total. Where that is not available, quote the count of photos and videos in the export and the gigabytes of the downloaded files.
What to enter on the form
Google Photos in GB, and the year your company started using it. Seller form