I have both. They both actually leave about the same effect on me. Neither are as ergonomic as a Para-2.
Both of them are extremely well-constructed. The pivots are both slick as snot, the lockup on both is vault-like with zero movement, which is very nice. It's rare to find knives that are this slick but also have zero lateral or front-back movement in the pivot or lock. Both of them are perfect in this regard. They are both just really, really well-made. Note that my Starmate does not have the rattling lanyard tube. The blade on the Starmate is very thick (4mm) while the GB is relatively thin, especially since it's hollow ground.
There are subtle differences that make them very different knives. The thin steel liners and thin G10 make the Starmate feel fairly thin in the hand. But because it's completely flat, it feels a little bit chunky in your hand. The contour cutout for your finger grip isn't deep enough to make it extremely ergonomic feeling in your hand. The best grip is a fist with your thumb on the jimping. The liner lock makes a solid "thwack!" sound like the lock on a Military. The G10 is very rough textured. The blade on the Starmate is flat-out impressive; it's thick, a great size, has a nice thumb ramp with jimping, has a great shape.
The GB has fairly thick carbon fiber scales, which make the knife feel relatively fat in the hand. Its edges are radiused so even though it's fat, it doesn't feel chunky. Again, the contour cutout for your finger grip isn't deep enough to make it extremely ergonomic feeling in your hand. The most natural grip is a three-finger modified fist grip with your index finger extended into the false choil and thumb on the jimping. The lock on the GB isn't as sticky as the Starmate, probably because all the surfaces and sharp corners were polished down. The carbon fiber texture is very smooth with barely recessed twill pattern. The M4 steel responds extremely well to a simple stropping. It came very sharp out of the box. Normally I will reprofile a pocket knife to 17-19 degrees per side with the EdgePro and then strop it to polish the bevel to scalpel sharpness. After a few passes on the strop with some pink compound the M4 blade was already scalpel sharp, no sharpening required. I was very impressed with that. It looks like it will be very, very easy to maintain.
I like both knives a lot. I really couldn't pick one over the other but they both are quite distinct and stand on their own merits. This is a situation where I really can say the advice to just get both is probably justified.