Panel from Fantastic Four #271 (October 1984); script; pencils, and inks by John Byrne; colors by Glynis Wein, letters by Janice Chiang
This tale of extraterrestrial invasion is set before the events of Fantastic Four #1, so it firmly falls within the Atlas Age of Monsters, a saga of a big space creature wearing big stretchy underpants, that might have been a feature in Tales of Suspense or Journey into Mystery, but starring the first superheroes of the Marvel Age before they got their powers!
Just as in the Marvel Age, Reed has no time to explain his plan...only time to talk about how he can't explain his plan!
Y'see, Reed had figured out that the alien could increase his size to attack the Earth, but not his mass. The Power Beamer beamed rays of sheer, delicious, nougaty power at Gormuu! But wouldn't (as Ben Grimm objected) just make him larger?
Why yes. Yes it would. It would increase Gormuu's size but not his mass until he grew so huge and so insubstantial that he just, like General "Happy" Sam Sawyer...faded away.
Well, then, that's a pretty handy invention there, Reed. Have you ever thought of making it a little bit smaller, giving it a much sillier name, and then using it again?
Why, yes! Yes, he has. Tune in tomorrow!