Why does the gửi sound like a G sound, should it be a Z sound?
The G sounds like G. But if there's an I after it, then it has a Z sound.
Is that just Northern pronunciation or also apply to central and south?
If it's Southern accent then "gi" sounds like a "y" as in z'yes".
It should be /ɣ/ (velar voiced fricative), both in northern and southern accent.
It is similar sound with english letter G but pronounced softer.
It's actually like G in amigo in spanish.