Gerard mentions:
The Hippie Trail: https://en.wikipedia.org/wiki/Hippietrail
Gerard Aartsen's blog about his travels on the Hippie Trail: https://gaartsen.home.xs4all.nl/0prologue-eng.html
Chariots of The Gods by Erich von Däniken: https://www.goodreads.com/book/show/91202.Chariots_of_The_Gods
Timeless Earth by Peter Kolosimo: https://www.goodreads.com/book/show/6012482-timeless-earth
George Adamski: https://www.the-adamski-case.nl/
Alice A. Bailey: https://www.lucistrust.org/books/about_alice_a_bailey
Benjamin Creme: https://www.share-international.org/background/bcreme/bc_main.htm
Maitreya: https//www.share-international.org/maitreya/Ma_main.htm
Pioneers of Oneness by Gerard Aartsen: http://www.bgapublications.nl/publications/book/2020-pioneers-of-oneness/
Lessons in Gnani Yoga - the Yoga of Wisdom (1909) by Yogi Ramacharakra: http://www.biblioteca-ga.info/50/toon/214/34.%20Yogi%20Ramacharaka
Operation Survival Earth by Stefan Danaerde: https://stefandenaerde.nl/2020/03/04/stefan-denaerde-operation-survival-earth-and-universal-creation/
Pioneers of Space by George Adamski: https://books.google.de/books/about/The_Pioneers_of_Space.html?id=ft_KtAEACAAJ&redir_esc=y
PDF of The Amazing Mr. Luttersworth by Sir Desmond Leslie: https://www.scribd.com/document/56673124/The-Amazing-Mr-Lutterworth-Desmond-Leslie-George-Adamski-1958
The Boy and the Brothers by Swami Omananda: https://books.google.de/books/about/The_Boy_and_the_Brothers.html?id=zw0vAAAAYAAJ&redir_esc=y
Helena Blavatsky: https://www.theosophical.org/component/content/article/25-online-resources/online-leaflets/1796-hp-blavatsky-and-her-writings
J. Krishnamurti: https://jkrishnamurti.org/about_landing
Helena Roerich: https://en.wikipedia.org/wiki/Helena_Roerich
Our Elder Brothers Return – A history in books: http://www.biblioteca-ga.info/