It was his idea originally apparently.
Toei did not give him any credit and he sued. Nagai has since gone on to say he is originally creator.
It was a thing they did back in the day. You had a good mangaka. You had an anime company. The two came up with a good idea and created a new anime at the same time the mangaka made his own manga of that anime. The two worked to promote the other. GAIKING may have been one of the ideas bounced around between Nagai and Toei though I am not certain of the details.