If GL and Gunbuster were set in the same universe, they still wouldn't be even closely related.
Gunbuster takes place in the present day.
Diebuster takes place in 14,200~
GL takes place millions of years in the future.
And, as I said a while ago, it wouldn't be that hard for Gainax to connect all their shows to one universe using vague stuff. Pic related (official book by gainax).
For example, the First Ancestral Race from Evangelion. Millions of years after giving life around the universe, and it going out of control repeatedly, the FAR decide to change what they're doing (by which point, humans have already discovered Spiral Power, once called Topless power), and become the Anti-Spirals to try and destory all life.