No, it wouldn't be like flamethrowers. Assuming you could actually get a weapon like that to work, it would be like putting someone in a microwave.
Which would be terrible, sure, but frankly, not a lot worse that what we already do to people on the battlefield, assuming the energy was high enough to kill them fairly quickly, which is the only way this weapon would be effective. However, assuming you could do this (and it would be hard) the real problem would be that pretty much nothing STOPS gamma rays, so you really couldn't control this weapon very well. You would be toasting ANYTHING in a line for quite a long distance.
Given the massive amounts of energy that would be required, I can't see this really being practical. If you have enough focused energy to produce a lethal gamma band, you have enough to melt that tank where it stands. The only benefit to gamma rays is that I guess you could theoretically recover the tank afterwards, though all the electronics would be fried. And you'd have to clean it out.
Yuck.
