Alrighty here you go:
The Producer Takeda says that they were thinking of making a movie from the time the TV series started airing. After it aired, people were saying they wanted to see more, and they had confidence that this work could be handed down for 10 years like Eva. But when he talked about it to Director Imaishi while the TV series was still going, he was told "No way!"
So what is the movie about? They were first thinking of making it a summary of the TV series, but don't intend to just rearrange it and end it like that. The theme is "revival." Becoming resolved, Imaishi said "I'll show you something that is Gurren Lagann but not Gurren Lagann!" and began writing great plans, but of course they were turned down. Although, they want to show everyone something very different from what they'd expect.
Also, currently they're in the middle of working on a completely new production different from the movie. They can't reveal the names of the participating staff yet, but he can say that they are distinguished members. They will screen a portion of this at the Tokyo International Anime Faire. "With the movie, we want to betray everyone's expectations in a good way. We'll reveal even more news at the Anime Fair, so please come watch."
The movie will be coming out in Autumn.