For those curious about the concepts behind Gerbil and its underlying algorithms, we recommend the scientific article A Novel Framework for Interactive Visualization and Analysis of Hyperspectral Image Data. It is published open-access, which makes it freely available.
If you use the software in your work, please include the citation:
Johannes Jordan, Elli Angelopoulou, and Andreas Maier, “A Novel Framework for Interactive Visualization and Analysis of Hyperspectral Image Data,” Journal of Electrical and Computer Engineering, vol. 2016, Article ID 2635124, 17 pages, 2016. doi:10.1155/2016/2635124
Spanish speakers might also be interested in the student thesis Procesado de imágenes hiperespectrales by Esther Sánchez Bernabé, which gives directions about how to use the software. Note that I do not speak Spanish myself, so I cannot assess the document.