If you liked the Hyakujuu-Ou GoLion anime, the Anime-Planet community thinks you'd like:
In the 21st century, the evil faction known as Galactor is set on world domination… and there’s only one group of people who can stop it: the Science Ninja Team! Under the guidance of Dr. Nambu and the International Science Organization, the team handles everything from taking down giant turtles to battling iron monsters, all in the name of justice! Go go Gatchaman! Take down the evil Galactor and save the world!
How does one start the similarities?
A team of 5
They fight against overwhelming odds.
Both have a hero vehicle of sorts. In Golion(voltron) It is 5 armored mecha resembling lions, that can combine into a giant singular humanoid mecha.
In Gatchaman (Battle of the Planets) the team has one giant singular ship that has mini vehicles inside of it (such as a smaller fighter, a rolling 4x4 and a motor bike)
both series the cast is dressed in a Ccrayola box of colors.
part of a classic genre of episodic action tales. Both are very similar.
There is a web of shows that have a hero machine, but these two share more in what they fight.
Both titles have large beasts that the hero mecha has to fight, that are more on the magical side.
In Golion it is more a team, in Koutetsu it is more a pair. But there was a combination sequence much like Golion(Voltron). Except the pilot in Koutetsu who actually transforms into the head.
I originally saw Koutetsu as (El Vegador) and Go Lion as Voltron, but the similarities are very much the same.