Not an astophysicist, but I can give a lower bound: the lower limit limit on beam divergence angle is (wavelength) / (𝜋 × (initial diameter)). Wikipedia suggests a source diameter of ~60,000 km, and the peak photon energy for the event was 18 TeV, which translates to a wavelength of about 7×10^(-20)m, putting the lower limit on divergence (for a perfect laser) at 7×10^(-20)/(𝜋 × 60000000) = 372 nanoradians, which gives a final radius at that range of 60000km + 2 × sin(372 nanoradians) × (2.4 billion light years), which works out to somewhere in the region of 1,800 light years. This beam was presumably a very long way away from being a perfect laser, and most of the particles will have had lower energies, so that's probably an order of magnitude or several too low. However you slice it, though, that's a pretty wide end target, so it's probably more accurate to say it hit our vague region of the galaxy, rather than that it hit Earth. Certainly it wasn't at risk of hitting the wrong bit of the solar system.