Alyx pretty much sums up the use of the gravity gun for you. "We usually use it for heavy lifting, but I've found it handy for clearing minefields."
So it's pretty much a portable forklift, with some limited military applications (other than the whole 'everything is a projectile' tactic, which I can see would cause a lot of problems in a real life combat scenario).
The Portal gun, on the other hand... well, it's a fucking PORTAL gun, think of the things you could do with it! We could make huge advances in science, transport, military hardware etc. or you could just use it to get home quickly.
Of course, mass production would be out of the question. Imagine the chaos an extremist organisation could cause with it, or even a small time assassin with friends in high places.
So I guess the Portal gun would be much more versatile, but the gravity gun doesn't have potential to screw the world over.