10 grams of antimatter will annihilate 10 g matter, which just produces 429.6 kT. Still a big bang.
Antimatter annihilation from anti-hydrogen is surprisingly messy: it will not be pure gamma rays. The positrons will meet electrons and produce 0.511 MeV gammas, but the protons meeting antiprotons will initially have a quark annihilate an antiquark, producing a gluon that then gets involved in messy hadronization leaving a bunch of mesons (pions and kaons) that then careen away and decay into muons, electrons/positrons, gammas, and neutrinos. The neutrinos will carry away a fraction of the energy but the rest will heat the vicinity into a fireball.
When the antimatter starts reacting antiprotons and positrons will be kicked away, mixing with the air. However, the mean free path is so short at ambient pressure that they will react before going far (doing it in the upper atmosphere might produce a much larger and fuzzier fireball). The gammas then scatter of air molecules, transferring the energy into heat. To some degree this is just like a normal nuclear explosion of the same yield.
Some of the radiation will doubtless cause fission or transmutation - a nucleus hit by an anti-proton is likely to at least lose a nucleon, and the mesons also happily react with nuclei. Sufficiently big detonations can presumably force a bit of fusion, but it is unlikely to be self-sustaining on its own without confinement.
The damage done to the biosphere here is more due to pressure and heat than radioactivity. If you have an absurdly large fireball it will tend to spread more upwards (less pressure, longer mean-free path) and send a big plasma cloud up - bad from an electromagnetic pulse, ozone layer and IR energy igniting stuff perspective.
Antimatter is not magic.