Although i'm stuck on v12.5 in Rapidroid, GreKo is updated to 13.1 for Andoid by Jim Ablett. Previous 12.6 and 12.8 compiles were hanging randomly during my engine tournaments. I wish 13.1 doesn't repeat this, so that i can rate the most recent one.
Under Android, use GreKo only in Winboard protocol mode. Therefore only with Chess for Android, since it's the sole GUI compatible with it.
Usage of the ini file and the book is not mandatory. However, the ini file works and can allow selecting higher hash sizes at least.
Downlaod GreKo 13.1: HERE