GBMB
https://github.com/simtr/The-Powder-Toy/blob/master/src/simulation/elements/GBMB.cpp
GOO
https://github.com/simtr/The-Powder-Toy/blob/master/src/simulation/elements/GOO.cpp
list of elements
https://github.com/simtr/The-Powder-Toy/tree/master/src/simulation/elements
i wasted so much time to find the source code for elements because i want to make liquid hydrogen :D and learn
Edit: VIP note :p
in the third link of all elements not all elements are named the same for example
LOXY named LO2 so be attention