Mixolydian G is quite simple really, it only uses major notes in this order:
G A B C D E F G
A little insight into the whole scale thing, half steps and full steps. Full steps appear after notes G, A, C, D and E. Its basically sharp and flat notes. Half Steps appear after notes B and E, these are notes that don't have sharps. If that makes sense read on, if not go to the next paragraph.....Half steps are more or less what creates a scale. With mixolydian scale half steps appear after 3rd and 6th notes and thats how those patterns are created. So to play mixolydian G scale in second position, i.e, starting from G on 3rd fret on top E string, you'll need to play half-steps after 3rd and 6th notes.
Now, how does it all fit into Chinacat? Well, Bobby does sort of play G mixo scale in a sense that he plays all the major notes apart from that C# on 15th fret on B string but that, I think (and correst me if I am wrong guys) is due to arppegios. Arpeggio is basically playing a chord note by note, and in this case I think its to do with A chord. A chord is made of A C# E, but C# doesn't figure in G mixo scale and is instead replaced by C. playing A C E instead of A C# E gives you A minor instead of A major, so my guess Bobby plays C# to make the song sound happier.....