24.16.1. gem5 debug build
How to use it in LKMC: Section 23.8, “Debug the emulator”.
If you build gem5 with scons build/ARM/gem5.debug, then that is a .debug build.
It relates to the more common .opt build just as explained at Section 23.8, “Debug the emulator”: both .opt and .debug have -g, but .opt uses -O2 while .debug uses -O0.