Yamato Takeru

The protagonist lives in a world where humans are no longer dominant and live in simpler times. The hero finds a buried mecha in the ground uses it to defeat an enemy that humans have never seen before. The mecha is originally meant to be a fighting machine for the enemy, but is found by the hero just in time. Themes such as overcoming impossible obstacles, despair, a great journey, betrayal, and love are similar. Both have big doses of comedy, but Yamato Takeru is more lighthearted and not as much fanservice. -SPOILERS- Other large similarities include both having mysterious extraterrestrial antagonists who command a group of elite beast-looking soldiers, each having a unique and powerful mecha. Add to the fact that one of these soldiers forms a heavy rivalry with the hero and in the end becomes an ally. Both have the hero meeting new friends along the way who join in the journey to reach the enemy stronghold. Also, it seems the hero's mecha in Yamato is very special much like Lagaan in Tengen Toppa. They change form throughout the series and end up combining with an enemy mecha to obtain massive power. The heroes' love interests end up becoming the enemy and they are forced to make tough decisions in the conclusions of both series. read more 

Both feature semi-sentient giant robots left behind from a forgotten era. The mecha designs also have some close similarities. 

They're quite similar. Both are mecha, both have young protagonists, both contain elements of romance, main robots in both series behave autonomously sometimes.