Gunbuster first makes Diebuster's ending really BAAAAWW and bittersweet.
Otherwise, the two are not too related until Episode 4 or so.
Basically, you should watch both in order.
It's not rehashing, actually. As you know, TTGL wasn't released until 4 years or so after Diebuster came out.
The whole PIERCE THE HEAVENS sort of mood actually came in with this series, so if anything, it's the other way around.
However, the character designs and art style are similar to both Eva and FLCL. Done by the same people, you know.
It's extremely awesome and one of the best anime, but it's way underwatched and underrated for how good it is.