I thought I'd have a definite favorite, but they're each different, and each are really nice sounding guitars.
Just so I'm comparing apples to apples, I put new D'Addario EJ17s on all of 'em, and tuned 'em to: C#, E, G#, C#, G#, C#, a new tuning I've been fooling with lately.
The GF30 is similar in sound to the F50 (not surprising -- both are archback models, maple sides and back), but I seem to prefer it to the F50. (I A/B'd the GF30 and my F50 a few years back and sold my F50.)
The GF50 accentuates the mid-range, while the GF60 seems to have slightly more pronounced trebles, and *maybe* more bass, though I'm not sure about that. It may be that the pronounced mids are tricking my ear.
Glenn