Expanding on John Robinson's answer, there are quite a few difficulties that the proposed Gamma Ray device will have, making it's use different fem what you seem to be proposing.

First off, we see the amount of energy required to power the device, and the short time periods involved in creating the gamma ray burst or beam. This means while the device itself might be thee size of a Dr Who Dalek, it will need to be attached to something capable of providing massive amounts of energy and support (such as cooling). An aircraft carrier might be an appropriate vehicle to carry one of these.

The second thing to consider is that gamma radiation, and indeed high energy radiation beyond ultraviolet is rapidly absorbed by the atmosphere, limiting the range of the gamma ray device. If I can move into range with an artillery piece or long range guided missile and strike the emitter before I am in the range of the gamma rays, then the gamma ray weapon is essentially nullified. Even if the gamma rays are of sufficient power to fry electronic guidance systems or prematurely detonate warheads, all I have to do is fire lots of "dumb" rounds and saturate the area where the gamma ray weapon is located. A concrete practice round will still strike the emplacement with a lot of energy and damage or destroy the delicate internal mechanisms of the gamma ray emitter.

Looking at the problem fem this angle, I can see a modification to your assumptions is needed, but fortunately(?) will still have much of the same effect. There are already devices called "wake field accelerators" (see: https://portal.slac.stanford.edu/sites/ard_public/facet/Pages/rpwa.aspx, and http://www.slac.stanford.edu/pubs/slacpubs/3750/slac-pub-3891.pdf, for example) which can accelerate beams of electrons in the space of a fraction of a metre.

These high speed electron beams are the heart of "Free Electron Lasers" (FEL) which have high "wall plug" efficiency (up to 60% of the energy can be converted to laser light) and are "tuneable" to frequencies that you desire. The FEL can be tuned to fire through one of the "windows" in the atmosphere which are not highly absorbent of laser light; the primary stipulation is you keep the beam energy low enough to prevent the atmosphere from breaking down into a plasma and absorbing the beam. A rapid pulse rate reduces this problem and also applies high energy shocks to the target, so in addition to thermal damage and the erosion or vaporization of the target material, mechanical shocks will also be weakening the structure.

Expanding on John Robinson's answer, there are quite a few difficulties that the proposed Gamma Ray device will have, making it's use different from what you seem to be proposing.

First off, we see the amount of energy required to power the device, and the short time periods involved in creating the gamma ray burst or beam. This means while the device itself might be the size of a Dr Who Dalek, it will need to be attached to something capable of providing massive amounts of energy and support (such as cooling). An aircraft carrier might be an appropriate vehicle to carry one of these.

The second thing to consider is that gamma radiation, and indeed high energy radiation beyond ultraviolet is rapidly absorbed by the atmosphere, limiting the range of the gamma ray device. If I can move into range with an artillery piece or long range guided missile and strike the emitter before I am in the range of the gamma rays, then the gamma ray weapon is essentially nullified. Even if the gamma rays are of sufficient power to fry electronic guidance systems or prematurely detonate warheads, all I have to do is fire lots of "dumb" rounds and saturate the area where the gamma ray weapon is located. A concrete practice round will still strike the emplacement with a lot of energy and damage or destroy the delicate internal mechanisms of the gamma ray emitter.

Looking at the problem fem this angle, I can see a modification to your assumptions is needed, but fortunately(?) will still have much of the same effect. There are already devices called "wake field accelerators" (see: https://portal.slac.stanford.edu/sites/ard_public/facet/Pages/rpwa.aspx, and http://www.slac.stanford.edu/pubs/slacpubs/3750/slac-pub-3891.pdf, for example) which can accelerate beams of electrons in the space of a fraction of a metre.

These high speed electron beams are the heart of "Free Electron Lasers" (FEL) which have high "wall plug" efficiency (up to 60% of the energy can be converted to laser light) and are "tuneable" to frequencies that you desire. The FEL can be tuned to fire through one of the "windows" in the atmosphere which are not highly absorbent of laser light; the primary stipulation is you keep the beam energy low enough to prevent the atmosphere from breaking down into a plasma and absorbing the beam. A rapid pulse rate reduces this problem and also applies high energy shocks to the target, so in addition to thermal damage and the erosion or vaporization of the target material, mechanical shocks will also be weakening the structure.