Not super simmilar to Gunbuster but Dangaioh and Fight! Iczer-1 are a couple of sweet 80s OVAs, and they're only three episodes each.
Getter Robo Armageddon is great as the other guy said
The King of Braves GaoGaiGar and the OVA sequel FINAL are fucking amazing super robot goodness.
Also Shooting Star Gakusaver is a sweet 90s OVA that's not talked about enough.
Also speaking of Getter, specifically in relation to Gunbuster, the manga may interest you, specifically Getter Robo G since Gunbuster has a few nods to it (original>G>Go>Shin>Arc is the manga order).
Getter OVAs are each their own thing and are mish mashes of various parts of the manga, so you may wish to read the manga first but if you're more keen on anime or whatever you'll be fine if you don't.