  GD-1 is a long, thin, Milky Way star stream that has readily visible density
variations along its length. We quantify the locations, sizes and statistical
significance of the density structure, i.e. gaps, using a set of scaled
filters. The shapes of the filters are based on the gaps that develop in
simulations of dark matter sub-halos crossing a star stream. The high Galactic
latitude 8.4 kpc long segment of GD-1 that we examine has 8+/-3 gaps of 99%
significance or greater, with the error estimated on the basis of tests of the
gap-filtering technique. The cumulative distribution of gaps more than 3 times
the width of the stream is in good agreement with predictions for dark matter
sub-halo encounters with cold star streams. The number of gaps narrower than 3
times the width of the GD-1 stream falls well below the cold stream prediction
which is taken into account for the gap creation rate integrated over all
sizes. Simple warm stream simulations scaled to GD-1 show that the falloff in
gaps is expected for sub-halos below a mass of 10^6 M_sun. The GD-1 gaps
requires 100 sub-halos >10^6 M_sun within 30 kpc, the apocenter of GD-1 orbit.
These results are consistent with LCDM sub-halo predictions but further
improvements in stream signal-to-noise and gap modeling will be welcome.
