Did a gamma ray burst contribute to the Ordovician mass extinction?
As a preamble, let me say that I don't know remotely enough on galactic dynamic to know if a supernova could have possibly been close enough during the Ordovician for Earth to be affected by a gamma ray burst, nor do I know enough about geochemistry to know if there are ways to detect such an event in the fossil record.  

That being said, the questions that I'll try to answer are: do we need to invoke an extraterrestrial event to explain the Late Ordovician glaciation? does the taxa selectivity of the extinction event support the GRB hypothesis?  

So let's look at the different hypotheses on how the glaciation occurred, the original conundrum being: how could a glaciation occur when mid-Ordovician $p_{CO_2}$ is believed to be up to 14 times Quaternary level? Brenchley et al. 1994 suggested that the Ordovician ocean circulation changed from a system having warm saline bottom waters to a system with cold bottom waters (i. e. a thermohaline instead of an haline circulation), which triggered a phytoplanktonic bloom, which then decreased atmospheric $p_{CO_2}$ and provoked an eutrophization of the ocean. Kump et al. 1999 argued in favor of a tectonically derived $p_{CO_2}$ decline: with the Appalachian and Caledonides orogenesis came an increased weathering which decreased $p_{CO_2}$. Of course the two hypotheses are not mutually exclusive since increased weathering also means increasing nutrients in seawater and therefore increasing primary productivity.

Another question is which groups went extinct during the glaciation: recent studies show that acritarchs (the most diverse group of phytoplankton known for that period) did not suffer from these events (Vecoli 2008); oppositely benthic group such as crinoids or brachiopods did (see Sheehan 2001 for a review); some pelagics groups such as graptolites and nautiloids did suffer as well during the event while some other didn't (radiolarians for instance). In summary there is no clear pattern, but there is also no reason to think that there was a preferential extinction of surface organisms over deeper-living ones.

So, does that answer the question: no; but it seems to me that GRB doesn't explain better the event that unfolded during the late Ordovician than other hypotheses (namely the ocean eutrophization and the continental weathering). Does it discard the hypothesis of a GRB: no; but since there is no direct evidence that it indeed happened at that time, and that we can explain the Ordovician events without, it seems, to me, more parsimonious to think that it didn't.

References:
Brenchley et al., 1994. Bathymetric and isotopic evidence for a short-lived Late Ordovician glaciation in a greenhouse period. Geology, 22: 292-298.
Kump et al., 1999. A weathering hypothesis for glaciation at high atmospheric $p_{CO_2}$ during the Late Ordovician. Palaeogeography, Palaeoclimatology, Palaeoecology, 152: 173-187.
Sheehan, 2001. The Late Ordovician Mass Extinction. Annual review of Earth and Planetary Science, 29: 331-364.
Vecoli, 2008. Fossil microphytoplankton dynamics across the Ordovician–Silurian boundary. Review of Paleobotany and Palynology, 148: 91-107.