Gene Roddenberry wrote lyrics to the Star Trek theme song, but he did it for kind of an asshole reason.

Without Courage's knowledge, Roddenberry wrote lyrics to the theme, not in the expectation that they would ever be sung, but in order to claim a 50% share of the music's performance royalties. Although there was never any litigation, Courage later commented that he considered Roddenberry's conduct unethical. Roddenberry was quoted as responding, "Hey, I have to get some money somewhere. I'm sure not gonna get it out of the profits of Star Trek."

source: https://www.lyricsondemand.com/tvthemes/startreklyrics.html

I did some googling, the only thing that I can find about Roddenberry and Earth Final Conflict is a three page "pitch" that's basically "what if aliens came to modern Earth and secretly enslaved everyone by literally mind controlling leaders and important people?"

Oh, and it had a different name.

So yeah. Not much involvement at all.

There were much better Roddenberry series ideas I’d like to see revived, like Assignment Earth.