Hi Travis,
As you say, ordinary atomic transitions don't have nearly enough energy to create gamma ray photons. Gamma ray photons tend to be produced during nuclear decays, which can release a lot more energy than electronic transitions. (You can also get gamma ray-photons from some other high-energy processes, like lightning. See the wikipedia page for for more.)
In general, photons can come from a huge variety of sources. Photons are just an excitation of electric and magnetic fields, so all you need is a process to provide energy and a means of converting another form of energy into light. Here are some examples.
Any accelerating electric charge, such as electrons in a wire, radiates light waves.
Another source of photons is from various particle interactions; as particles are destroyed or created, photons can be released as by-products.
Even imploding bubbles in a liquid can create light (see sonoluminescence on wikipedia).
In addition, photons can be split into multiple lower-energy photons, and the energy of a photon can be increased by, for example, bouncing it off of a moving mirror. Light can also gain or lose energy by scattering off atoms or materials in a non-resonant way (think deflected, not absorbed or emitted).
Hope that helps,
David Schmid
(published on 03/02/2014)