Though you could compare such sites based on: - individual title design (cover, ebook formatting/layout) - curation of titles and authors based on genre, literary period, country, and other factors
Legally, books are freely available where public domain status for a book is recognized. In the U.S., that is practically anything published before 1925. In most other countries, it's 70 years after the author or translator's death.
Gutenberg decided to block access to its main site in Germany (where I am), to resolve copyright claims.
Australian law says life + 50 years for authors that died before 1955. That'd make Orwell's work in the public domain since 2001 in Australia, and in the US starting from next year (died in 1950 + 70 years). For convenience copyright for those that died 70 years ago expires on January 1st next year, not exactly 70 years ago, so the author that died on January 2nd 1950 an author that died on December 31st 1950 enter the public domain at the same time: January 1st, 2021.
So commercial use actually requires that you strip Project Gutenberg's trademarks and redistribute only the public domain text.