Is 'guts' singular or plural? Like, which is correct: "Guts are required for this" or "Guts is required for this"?
It could be singular or plural depending on the context. "Gut" and "guts" have several different meanings which could be used in the OP's sentence.
If "guts" means "the entrails of an animal, removed by a butcher" or "personal courage or determination" then it is plural - "Guts are required for this".
IF "gut" is a mass noun meaning "Fibre made from the intestines of animals, used especially for violin or racket strings or for surgical use." then like all mass nouns it is singular - "Gut is required for this."
Reference for meanings of "gut(s)" : https://en.oxforddictionaries.com/definition/gut #s 1.2, 3, and 4.
“Guts” is used widely used informally especially in British English to mean fortitude, courage or determination:
informal the courage and determination you need to do something difficult or unpleasant
It takes guts to start a new business on your own.
have the guts (to do something)
No one had the guts to tell Paul what a mistake he was making.
The plural designation is also shown in the entries in Merriam-Webster, Collins, and Oxford Learner's dictionaries.
The cited use of guts is exactly paralleled by brains. We tend to think of the "idiomatic" senses (courage / intelligence) as only applying to the plural form, because we never use He has gut / brain to mean He's brave / clever.
But noting that many if not most native speakers would prefer Guts / brains is something he's never been short of, rather than ...are something..., I think it's reasonable to say that semantically, we think of the idiomatic plural usage as representing a singular quality / attribute.
It's almost meaningless to ask which version is "right", and Anglophones in general have a long history of ignoring pedants anyway (consider the data is vs the data are :). But unquestionably I personally would favour semantics over syntax in OP's case.
EDIT: Thanks to @Ronald Sole for the link to McMillan Dictionary..., wherein their example usage That’s what you need to be a referee – guts clearly uses a singular reference (that is what you need, not those are what you need).
It may not be directly relevant to OP's exact example, but when searching for relevant pronouncements on the usage, I came across this delightfully ambiguous example...
These animals have brains Semantics: A Reader (2004) - Page 420
...which could be literal - each of the animals actually does have at least some kind organ containing neurons. But it could also be figurative - some or all of them are [unusually] smart. And exactly the same ambiguity would apply with guts in that example (digestive organ / bravery).