Complex sentence: Despite the Gyaos being destroyed, Nagamine warns that more Gyaos may appear if the pollution of the environment continues, but Asagi assures them that Gamera will return if the human race ever needs him again.
Split into simpler sentences: Despite the Gyaos being destroyed, Nagamine believes more Gyaos may appear if the environment continues to strive the way it does. But Asagi assures them that Gamera will return if the human race ever needs him again.