Here’s the intro to the Gramps logfile article in the wiki:
A fresh Log file is created for each worksession in the application. (This overwrites any previous log since it uses the same name.) A file named Gramps51.log is created in the Gramps User Directory.
(The path differs by OS.) It is a plain text file.
PS: this was this 1st time I’ve found to use Discourse’s linkify terms feature for the logfile