Someone with a more comprehensive understanding of physics will hopefully give you a better answer but here's my probably incorrect guess: (I am going to butcher the shit out of this but I happily and genuinely encourage corrections.)

Gravity is incredibly weak. You can pretty easily defeat it by, like, lifting up your arm. Think about that- you, a puny little simian, can counter the gravitational pull of the entire planet by lifting your arm. Gravity only really becomes powerful by way of enormous mass and/or momentum. The more mass something has, the stronger its gravitational pull. Gravity can even act on light because light has momentum.

If you were somehow able to concentrate it into very strong coherent gravitational waves, assuming the laws of physics didn't melt down in that circumstance, (which they very well could?) it would probably act like a singularity except... in line form? It's possible the line could collapse into a singularity? It might destroy the very object generating it and collapse, resulting in an explosion of astronomic proportions.

Those are just my bad, bad guesses. I hope someone who is actually good at physics can give you a better answer. I'm more a bio and chem kinda gal. Never did as well at physics as I would have liked. Heh heh. ಠ‿ಠ;