The gravitational field of a cosmic string in 3+1 dimensions is simple - the spacetime is flat everywhere except for the locus of the string, but it has a deficit angle, much like a usual cone (multiplied by two more flat dimensions along the stringy worldsheet). The deficit angle is something like "8.pi.G_{Newton}.T" where T is the tension of the string - roughly the energy density per unit length of the string. The gravitational lensing by cosmic strings is special - the cosmic strings create two identical images, and they are visually separated roughly by the deficit angle itself. There are various upper bounds and lower bounds what the tension of the cosmic strings should be - i.e. how large the deficit angle of potential realistic cosmic strings can be if the cosmic strings exist. These upper and lower bounds marginally contradict each other (the uncertainty seems to be large enough so that the picture may still be consistent), but all these bounds are close to the deficit angle slightly below 10^-6. Because the tension has units of "squared mass" and because the deficit angle is roughly the tension in Planck units as we've said, we see that the mass scale associated with the string tension is roughly 10^-3 of the Planck scale - which is nothing else than the GUT scale. This fits the estimate for the tension that you expect from some GUT cosmic strings or the fundamental strings in string theory themselves - in the nearly old-fashioned models where the string scale is close to the GUT scale.