Hi
To see the gcc version used to compile the kernel is..
Code:
cat /proc/version
Linux version 4.4.74-92.29-default (geeko@buildhost) (gcc version 4.8.5 (SUSE Linux) ) #1 SMP Thu Jun 29 13:06:32 UTC 2017 (561ddb1)
As for the rest, I would assume it's the default gcc (4.8.5) but would use gcc6 for my own code...