"This has been going on for some time and the date of closest approach keeps getting pushed back. Here is the latest report from the New York Times on the approach of a gas cloud called G2 towards the Galactic Center.
Black holes, which are the ultra-dense, collapsed objects predicted by Einstein's theory of general relativity, are often depicted as voracious feeders whose extraordinary gravity acts like a one-way membrane: Everything is sucked in, even light, and virtually nothing leaks out.
Now, for the first time, astronomers may have a chance to watch as a giant black hole consumes a cosmic snack.
In March or April, the gas cloud G2, which has been hurtling toward the center of the Milky Way, is expected to collide with Sagittarius A*, a black hole that lies just 26,000 light-years away from Earth."
Almost certainly no gamma rays, although NASA's Fermi Gamma-ray Space Telescope is now operating in an observing profile that will give it good exposure to Sgr A* in case something does.
To get gamma rays, you usually need some highly relativistic particles, thus something to accelerate them. When objects fall into black holes, they can (though they don't always) convert a significant fraction of their rest mass energy into heat and light. However, that still falls far short of the particle energies you need to generate gamma rays. Instead, the hot matter glows brightly in optical and X-ray. The plasma movement through the strong magnetic fields of the disc can also generate synchrotron radiation, which peaks in the cm-mm radio band.
Some accreting black holes in other galaxies do emit powerful jets, which in turn emit gamma rays. When we see the gamma rays, the jets are pointed nearly directly at the earth, giving a strong relativistic boost to the signal.
reddit-style source: I am an astrophysicist working with Fermi and radio telescopes.