Squid sets the GID to the effective user's default group ID
(taken from the password file) and supplementary group list
from the groups membership.
If you want Squid to run with a specific GID regardless of
the group memberships of the effective user then set this
to the group (or GID) you want Squid to run as. When set
all other group privileges of the effective user are ignored
and only this GID is effective. If Squid is not started as
root the user starting Squid MUST be member of the specified
group.
This option is not recommended by the Squid Team.
Our preference is for administrators to configure a secure
user account for squid with UID/GID matching system policies.