If you're looking for manga similar to Great Mazinger vs Getter Robo, you might like these titles.
The manga begins with the Mazinger Z and Great Mazinger fighting against an alien machine after the Diana A and Venus A have been taken down.
After Emperor Gore and the Reptilian empire is defeated, a new enemy, Emperor Burai and his Hyakki empire rises and threatens to take over Earth. Ryoma, Hayato, and newcomer Benkei pilot the new Getter Robo G to save Earth.
In his skyless cavern of a village Simon toils daily, drilling holes to expand his stifling little world until one day he makes an extraordinary discovery: a small glowing drill-bit and the man-sized mech it activates. Before he can give it a second thought Simon's dragged into a plot to break through to the surface by the local gang leader Kamina, only to have the ceiling come crashing down on top of them under the weight of a giant monster! It somehow falls onto the boisterous Kamina and cowardly Simon to defend their village, but once they defeat the monster what awaits the duo on the surface world? Get ready for buxom babes, beastmen, and giant mechs as only GAINAX can provide them!