According to Wikipedia, gamma rays have a wavelength <10 picometers, which is smaller than an atom.
From what I've read, some around LPF, xray lasers are extremely difficult and impractical. I doubt anyone will produce a gamma laser anytime soon...
In terms of wavelength, gamma rays and x-rays are basically the same thing, i.e. photons with energy exceeding that of shortwave UV. The difference is in their origin.
By modern convention, x-rays are high-energy photons originating from processes and interactions involving electrons. Bremsstrahlung is one example, high-energy synchrotron radiation is another (although synchtrotron radiation can also be low-energy, e.g. microwaves, depending on how the device is set up.) The free-electron laser would, by this convention, produce x-rays rather than gamma rays, assuming the device is configured to produce photons of such energy.
Gamma rays are high-energy photons originating from nuclear transitions and reactions; photons arising from such processes are always termed 'gamma radiation' regardless of energy. While in most cases such photons will be more energetic than bremsstrahlung and other x-radiation (due to nuclear transitions being inherently highly energetic) this is not necessarily the case.
As an example, when a cobalt-60 nucleus decays it emits gamma rays with energies somewhat higher than 1 MeV. The photons are termed 'gamma rays' because they arise from a nuclear reaction, in this case a radioactive decay and subsequent relaxation of the daughter nucleus into its ground state.
On the other hand, the Edwards Accelerator Laboratory here at Ohio University can accelerate electrons up to 5 MeV energy, and if these are directed at a target, bremsstrahlung x-rays of up to 5 MeV can be produced. These photons are termed "x-rays" because they arise from the deceleration / change of motion of electrons, even though they are more energetic than the cobalt-60 gamma rays mentioned above.
I'm sure I left out some detail or another, but that is basically how x-rays and gamma rays are distinguished in modern physics. In common parlance, the older, energy-based distinction still predominates.