Hey Habanero,
I have both a GB and G2 except my GB is a earlier model before the final product we see now.
To answer your question the main difference that puts the GB above the G2 for me is the S rod configuration, Ive used that style for 14 years so its mainly personal preference. I find the S rod to be more sturdy and to tip less when set down as digging a target.
If I had to change anything as far as the disc goes it would not be a notch but instead to be able to expand or decrease the range of the high low tone, For me the low tone range shrinks to much at higher discs.
The other thing that is specific to the pre SE versions is the non Vco audio, though the g2 and GB pro have more personality and finesse as the final product I fell the 2.0 GB hits harder on small objects at depth, I would love to see a future model with the ability to switch back and forth from saturated audio to VCO. Though both detectors operate exactly the same their really is a performance addition to having saturated audio, so much so that in my opinion it would be like having two different detectors in one. (my little wish list)
HH