Similar to the original version of MiMC, GMiMC also uses the function f(x) = x3 as the main component of its construction. However, where MiMC uses a simple encryption path or, alternatively, a balanced Feistel network, GMiMC uses a different approach, namely unbalanced Feistel networks. The GMiMC family of block ciphers specifies various Feistel constructions, including, for example, the expanding round function, which has shown to be the most efficient one in most of our use cases. All details about GMiMC can be found in the paper.
Similar to MiMC, most of the versions of GMiMC using a round key of size n (i.e., a single key component) are vulnerable to an attack recently presented at SAC 2019, which does not depend on the specific round function, but rather on the key schedule being used. The full-key version of GMiMC is unaffected by this attack.