Gerster earned a PhD in German literature and philology from the University of Zürich.
Since 1956 he has been a freelance writer and photographer, specializing in science reports and frequently contributing to National Geographic, GEO, Paris-Match, and Neue Zuercher Zeitung. From 1950 to 1956 he was science editor of the Weltwoche, a Zürich-based weekly.
He has received the Anerkennungsgabe der Stadt Winterthur (1977), World Understanding Through Photography award (1976), Pictures of the Year award (1976), Prix Nadar in Paris (1976), Ehrengabe des Kts. Zuerich (1974), and Die Goldene Blende from Germany (1973).
His photos have been shown in single and group exhibitions in Europe, Japan and the USA. He was represented by the Rapho photo agency.
Gerster emphasizes, on the one hand, the beauty of the landscape; on the other hand he points out the endangerment to nature caused by excessive use, erosion, technology and mechanization. The fact that he does not only want to make "beautiful pictures" shows in his expert and precisely investigated captions and book texts.