I"m not exactly sure how they knew about "garebeg", but the event is documented in Grove's Dictionary of Music (Grove's, vol. 9, p.506).
A Javanese ensemble played at the Arnhem exhibition in the Mangkunegaran Palace in 1879 marking the first expedition of an entire gamelan ensemble to Europe. After that first appearance, gamelans slowly started to appear at exhibitions all across Europe and America. The next appearance I"m aware of occurred in 1882 at the London Aquarium and then again at the 1883 International and Colonial Exhibition in Amsterdam. America's first introduction to gamelan took place in 1883 at the Columbian Exposition in Chicago. This ensemble actually left their instruments to be kept at the Field Museum in Chicago.
It was the next appearance of gamelan in Europe that may have had the largest affect, making it the most notable. The year was 1889 and the place was the Exposition Universelle in Paris when Claude Debussy (amongst other composers) was first introduced to the sounds of the Orient and gamelan more specifically. The 27 year old fell in love with Asian music and started to incorporate Oriental melodies, scales (pentatonic), and titles (Pagodes) into his pieces. .
The first appearance of a Balinese gamelan orchestra playing in Europe occurred at the 1900 Exposition Universelle in Paris. It was just a little later that the long time Debussy student, Edgard Varese, began to also show an interest in Asian music. Varese, charmed by the sounds of the Orient, began to spread his ardency for the music amongst his American counterparts after emigrating to America. It was not the music itself that captivated Varese and other east-meets-west pioneers, but the realization that musical systems outside of the western twelve tone system could be employed to compose complex arrangements in a semi-western fashion. While none of Varese's compositions can be said to have a direct link to gamelan, it is important to note the influence that Asian music had on him.
