First, why are you trying to use --system? This implies that you have multiple users that need to share some baseline config. Is that really the case?
If not, then just drop the --system and let the change be placed into ~/.gitconfig or ~/.config/git/config whichever is the case for you.
If you do really need to use --system, then …
The reason you need to use sudo is:
bluefin(and probably the rest) do not ship an/etc/gitconfigfile/etcis owned by root
Go ahead and issue the command with sudo and then (also using sudo) change the file’s ownership as you would like.
The /etc filesystem is read-write. Adding files is fine and they will be propagated through system updates.
If interested, I wrote an article here that goes into more details.