Marx didn't endorse the labor theory of value either. That's a common misinterpretation. Or misunderstanding. Simply googling or wiki-ing will do.
Though if you want true socialism or communism, i.e. public and democratic control of production, you'd measure effort in a way that might
resemble this theory. It doesn't mean it's wrong historically, it's just a different economic concept that, according to the Marxian utopists, never
really was tried in practice (maybe the Spanish Anarchism came close).
And he did criticize "interest" all the time. He called it "Mehrwert" (surplus value). (He didn't differentiate it, and he was right with that, because there
should be no real difference between interest and dividends, today things are even more fucked up.) In fact that's what Marxism is all about.
(I don't endorse Marx, but it's not helpful to build upon false criticism).
btw and more on-topic again, Gesell's Freiland
is socialism, and my point of view is still that this is the big elephant in the room of his theories. If demurrage causes savers to flee into hard assets, they'll create bubbles. The same landgrabbing would happen that happens today with inflation. Gesell acknowledged that and called for "Freiland" (public/socialized land). But obviously, this will not be enough, you'd have to socialize everything then, the whole stock market, and finally the whole production. That's why Freiwirtschaft is not "ein dritter Weg", a third way (besides capitalism and communism), but rather an unstable compromise, imho.