#1
So ya I have an Sg-400. Every other string stays in tune except the g string which WILL NOT stay in tune at all. One bend and it's out. The guitar has grovers so it can't be that and the bridge saddle looks fine. That leaves the nut obviously, but looking at that as well it looks fine to my eyes but I could be wrong. Should I just go pop a new nut in or something?
#2
Check how the string is wound on the peg, the winding should be flat with no overlapping. Check that the peg is not out of position or leaning over. Check that the knob is not loose.

Loosen the string. Clean and lubricate the nut slot using graphite or pencil lead.
#3
gbaddeley gave good advice. If that doesn't work, try buying a wound G-string. Most packs come with an unwound G string.

For all my electric guitars, the g string went out of tune often (someone mentioned because I live in a cold country and my guitars get moved around a lot. don't know how true that is.) But when I started buying strings with a wound 0.17 g string, the problem went away.
#4
You need to ensure that the nut slots are not causing the string to bind. Lubricate the slots with a pencil. If that doesn't help, you may need to widen the slot slightly as the slot could be pinching against the string, causing it to go out of tune.
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