@Yazoo I have corrected my correction. B is the ‘leading note’ in the key of C.
@J.W.C You have it right.
G is ‘dominant’ and does pull back to the tonic because the leading tone has that effect.
Also, G dominant 7 pulls even more strongly back to the tonic chord because it has, in addition to the leading tone B, an F note which pulls strongly to the 3rd of the tonic chord - the note E.
Added to which, dominant 7 chords contain a tritone, are inherently unstable and scream out to resolve somewhere else.
Check the graphic showing two notes from the G7 both of which are just a semitone away from a chord tone of the tonic, giving that strong sense of resolution.
Cheers 
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