Hey there! This is a great question. Yes you are correct, if the key is G major, you'd expect that chord (the IV chord) to be C major. So why the heck is it a C minor chord?

The answer can be explained by something called "modal interchange" -- which is when you substitute a chord from a different key temporarily, then return to the original key. So, in this case, the songwriter chose to temporarily move to the key of G minor and use a C minor chord in that spot, then by changing to the D major chord, he returns to the key of G major.

Sometimes songwriters do this to accommodate a complex melody they might be hearing for the song. Sometimes it's simply just to spice things up. While most songs can stay completely in one key throughout, other songs roam freely by using "modal interchange" to produce unexpected or interesting results.

Hope this helps!
Mike