Could Gamma ray bursts be caused by matter-antimatter annihilation? As far as I know a collisions of matter and antimatter leads to the complete  annihilation of both, whereby 100% of the rest mass of the particles is converted into gamma rays. Could this mechanism be responsible for the Gamma Ray bursts seen by satellites? Could therefore Gamma Ray Bursts tell us something about where all the antimatter is in the universe?
 A: Believe it or not, this was actually a theory held back in the 1990's! Astronomers back then thought that GRBs were the direct result of anti-matter-matter collisions (between anti-matter comets and matter comets) that were taking place in the Oort cloud. 
This 1996 article by Chuck Dermer (paywall), titled Gamma-ray bursts from comet-antimatter comet collisions in the Oort cloud, discusses the details of how it could be possible.
Unfortunately, that theory has since been thrown out the window for a few reasons, but the most important one was the connection of supernovae and GRBs (arXiv link). The current thinking is that short-duration GRBs are caused two neutron stars or two black holes that are merging (spiraling around each other) while long-duration GRBs are caused by hypernovae (super-luminous supernovae) that produce black holes (which causes the explosion to go outwards in the commonly-shown jet emission, rather than a spherical explosion).
A: Here is a thesis trying to fit the spectra of the observed gamma rays, and if you read the conclusions you will see that there are various sources.
One has to distinguish antimatter as low in energy as a positron, which can also appear from decays of isotopes:

The observed distribution of gamma rays is consistent with the standard picture where the source of positrons is the radioactive decay of isotopes of nickel, titanium and aluminum produced in supernova explosions of stars more massive than the Sun,” said Rothschild.

Even though mundane explanations can model the observed spectrum people have in mind the possibility of the gamma ray burst to include antimatter annihilating on matter. Here is a talk from 2005 with some expected spectral distribution.
Baryonic antimatter would give energies of the order of MeV  and the searches include the possibility.  We have not had any exciting news on the front so I suppose they are gathering data on the high energy tail before coming to conclusions. At the moment they have seen positrons correlated with thunderstorms on earth!
