For any major played, (G major, D major, etc.) do we play everything else as a natural? (Ex. G major only uses every
F as an
F#, so would
G both be played as a natural in the piece if they are there?)
If you are in the key of G Major you only have one accidental: F# as you already know. You will see the # noted in the key so every F you find will be sharp if not differently stated. And again, if not differently stated, this means every other note will be natural.
But remember, a scale is only a reference, not a strict unbreakable rule. You can find other accidentals even without modulating to a different key. Those are used for a lot of reasons: building tension, supporting the harmony, bridging between two chords and so on..
So in the same piece, in the same G Major scale, the C (or even the G as every other note) can be used natural, sharp or flat.