  All quasars vary in their optical flux on a time-scale of years, and it has
been proposed that these variations are principally due to gravitational
lensing by a cosmologically distributed population of planetary mass objects.
This interpretation has implications for the observable properties of gamma-ray
bursts (GRBs) -- as a source expands across the nano-arcsecond caustic network,
variability is expected -- and data on GRBs can be used to test the proposed
model of quasar variability. Here we employ an ultra-relativistic blast-wave
model of the source, with no intrinsic variations, to study the effects of
nanolensing on GRBs. Taken in isolation the light-curves of the caustic
crossings are predictable, and we find that a subset of the predicted
light-curves (the image-annihilating fold crossings) resemble the ``pulses''
which are commonly seen in long GRBs. Furthermore, for sources at high redshift
the expected time between caustic crossings is of order seconds, comparable to
the observed time between pulses. These points suggest that it might be
possible to model some of the observed variations of GRBs in terms of
nanolensing; however, our simulated light-curves exhibit a small depth of
modulation compared to what is observed. This means that the GRB data do not
significantly constrain the quasar nanolensing model; it also means that the
simplest nanolensing model cannot explain the observed GRB ``pulses''. Viable
nanolensing models for pulses probably require a large external beam shear. If
a viable model can be constructed it would effect a considerable simplification
in source modelling and, ironically, it would explain why no macro-lensed GRBs
have been identified to date.
  (Abridged)
