A gimme idea is an idea that is so good, a comic would be pretty good just by the existence of the gimme idea.
In this regard, I have almost come to look at gimme ideas as almost lazy writing, in the sense that, since the idea is a gimme, the writer doesn't have to work that hard to make the story cool, because the idea itself is so strong.
A major exception to that rule, I think, is when the writer comes UP with the gimme idea on his or her own, instead of using someone else's gimme idea (or an obvious gimme idea - an example of an obvious gimme idea is famous heroes fighting each other). Then I think the writer would be silly NOT to use the gimme idea, as he/she came up with it themselves, so should be the one who gets credit for the idea.
An example of a writer using someone else's gimme idea would be Mark Waid using the gimme idea of Batman being prepared to take down the entire Justice League.
An example of a writer using a gimme idea of his/her own device would be Joe Kelly having Deadpool travel back in time, and having the past be an actual 60s issue of Amazing Spider-Man.