To give an example, "I like being able to fire people" or "the private sector is doing fine," are examples of gaffes. In the first instance, Romney was trying to illustrate the value of private sector accountability for healthcare providers instead of a government assigned provider. In the later, Obama was pointing out that his policies had, in fact, spurred significant private sector job creation and that much of the high unemployment rate was a direct result of public-sector cuts enacted or supported by Republicans as cost-cutting measures. Restated, you may agree or disagree but at least it doesn't sound irrational/awful/heartless/out of touch/chose your negative narrative.
Then there are statements that cannot be restated to make sense. They raise fundamental questions about the candidate's qualifications for office. For example, Michelle Bachman's statement that the HPV Vaccine causes mental retardation or Senate candidate Sue Lowden's suggestion that people could barter chickens and other things for health care. These are not gaffes. They are not simply misstatements of an idea that is defendable. No matter how you try to restate them, you look at them and go "can someone who believes that really be fit for the office for which they are running."
For my money, Tod Akin's comments on rape fall in the later category. This is not a "gaffe." It is not 'well, he shouldn't have said it, but it is really no worse than other statements by other candidates.' It's not about "forcible" v. "legitimate" or some other choice of words. The only response to this statement is "have you lost your mind? What on God's green Earth could make you say something like that?"