runtime.gogc configures GOGC (the GC target percentage).
runtime.gogc is equivalent to setting the
environment variable which also takes precedence over
the value from the config file.
Increasing this value means fewer but longer GC cycles since there is more garbage to collect.
NOTE - the default for fabio up to 1.5.14 was 800. This changed to 100 in version 1.5.15
The default is
runtime.gogc = 100