Ugh. Be careful with that, since actually giving a /g has the effect of toggling the behaviour all together.
From :help 'gdefault':-
:help 'gdefault'
'gdefault' 'gd' boolean (default off)
global
{not in Vi}
When on, the ":substitute" flag 'g' is default on. This means that
all matches in a line are substituted instead of one. When a 'g' flag
is given to a ":substitute" command, this will toggle the substitution
of all or one match. See |complex-change|.
command 'gdefault' on 'gdefault' off ~
:s/// subst. all subst. one
:s///g subst. one subst. all
:s///gg subst. all subst. one
While not quite documented under ":help :s_flags", two 'g' flags cancel each other out. This also applies to 'gdefault'.
Also, be aware that this may break a huge amount of scripts out there, should you decide to use it as some (hard to debug) issues are caused because of it.