  We present multiwavelength observations of the gamma-ray burst GRB 051028
detected by HETE-2 in order to derive its afterglow emission parameters and to
determine the reason for its optical faintness when compared to other events.
Observations were taken in the optical (2.0m Himalayan Chandra Telescope, 1.34m
Tautenburg, 4.2m William Herschel Telescope) and in X-rays (Swift/XRT) between
2.7 hours and 10 days after the onset of the event. The data can be interpreted
by collimated emission in a jet with a typical value of $p$ = 2.4 which is
moving in an homogeneous interstellar medium and with a cooling frequency
nu_{c} still above the X-rays at 0.5 days after the burst onset. GRB 051028 can
be classified as a ``gray'' or ``potentially dark'' GRB. On the basis of the
combined optical and Swift/XRT data, we conclude that the reason for the
optical dimness is not extra absorption in the host galaxy, but rather the GRB
taking place at high-redshift.We also notice the very striking similarity with
the optical lightcurve of GRB 050730, a burst with a spectroscopic redshift of
3.967, although GRB 051028 is about 3 mag fainter. We suggest that the bump
could be explained by multiple energy injection episodes and that the burst is
intrinsically faint when compared to the average afterglows detected since
1997. The non-detection of the host galaxy down to R = 25.1 is also consistent
with the burst arising at high redshift, compatible with the published pseudo-z
of 3.7 +/- 1.8.
