Most of the footage is from the episodes of the original episodes. (This is confirmed during the little montage after the group sets out to defeat the Beastmen). Most of the voice actors don't even sound like they're trying (IE Katsuyuki Konishi and Nobuyuki Hiyama don't sound as passionate as they did in the TV series. We finally get some new footage in the last 20 minutes of the movie and it was this entire movie should've been. New ways to make Gurren-Lagann look more badass than it already was. Viral gets his own Dai-Gunzan and becomes a General in Thymilph's place (I guess). Cytomander is killed by Simon's Giga Drill Break. Yoko fights Adiane in hand to hand combat. Kittan, Kiyal, and Kiyoh take down Guame. And then the four generals combine their Dai-Gunmen together to create an even larger Dai-Gun Doten-Kaizan.
After the hellstorm of missiles is fired upon the Gurren-Dan they all manage to stop it using a spiral shield and a Dai-Gurren Giga Drill Breaker to finish it off. And no, this is not the Dai-Gurren using the GDB. This is basically Gurren-Lagann creating a Giga Drill then after he flies towards the enemy everyone else fires towards the enemy, the attacks combine with the Giga Drill. Gurren-Lagann breaks into the ne Dai-Gun and all of the attacks that combined with the GDB fly off of the Giga Drill and attack the inside while Simon heads higher with the GD and kills the other Generals besides Viral, who lives.
So basically a good summary of the is it's the same shit until the last twenty minutes when something awesome finally happens.
Below is a picture of Doten Kaizan.