Every fantasy story and most sci-fi has something dark or grim in it. When does it hit the boiling point and become "grimdark"? When you feel like it does. Every reader has their own standards of dark and gritty. Alternatively, it's whether the AUTHOR says they're grimdark.
Still, I think this is a subject which deserves to be handled by a checklist of common factors which can tell you whether or not a work is grimdark or not? A work can be grimdark if it has all of these or none of these but this is some of my experience with the genre.
Take a shot for every "Yes" answer on the sheet. If you're drunk or dead by the end, it's probably grimdark.
1. Is war portrayed as a terrible thing where lives are senselessly lost in a futile struggle?
2. Is the central conflict one where there is no good or evil side? Just people?
3. Does the protagonist's side of the conflict ever commit war crimes?
4. Do the protagonists ever commit war crimes (torture, execution of prisoners, or worse)?
5. Is at least one of the protagonists someone who has done something unforgivably awful in the past?
6. Are idealists in the setting portrayed as painfully naive?
7. Are the cynics in the setting portrayed as factually correct?
8. Are romances likely to break up or fail versus true love or redeeming?
9. Are the romances unhealthy or obsessive in some way versus romantic?
10. Is sex just something people do versus romantic?
11. Are the nobility portrayed as mouth-breathing snobs with nothing redeemable about them?
12. Are the peasants portrayed as dirty superstitious ignorant louts?
13. Is the local religious system portrayed as hopelessly corrupt or a scam?
14. Is the government dirty and corrupt too?
15. Are the protagonists motivated by revenge, greed, or something wholly selfish?
16. Does the heroes triumphing make no noticeable difference to whether the world sucks or not?
17. Do they use real or made up curse words?
18. Is violence portrayed as visceral with bodily fluids and crippling versus neat little slashes or shots?
19. Do any of the heroes die?
20. Do any of the heroes die in a manner which is not heroic or dignified?
21. Do any characters have their ideals smashed painfully?
22. Is there no sign of beneficent supernatural forces?
23. Is the revelation of the world's mythology one where the gods are evil, indifferent, or nonexistent?
24. Is magic or psychic powers something which is innately evil or comes with a horrific cost like sanity or blood sacrifice?
25. Is there some historical but values incorrect practice going on in society like human sacrifice, forced marriage, or slavery?
26. Are the protagonists involved in any of these values incorrect practices?
27. Is acting in a selfish manner likely to reward heroes rather than punish them?
28. Is acting in a heroic manner likely to punish heroes rather than reward them?
29. Is prejudice against a religion, race, sex, or otherwise just a part of the setting?
30. Is said prejudice something the protagonists engage in without commentary or learning a lesson?
31. Are there prostitutes in a prominent role?
32. Do the protagonists engage in prostitution?
33. Are the protagonists likely to engage in adultery, drinking, smoking, drugs, or other vices?
34. Do the protagonists ever lose a battle or conflict?
35. Does the protagonists' side ever lose?
36. Do wounds ever get infected or characters maimed?
37. Is the ending bittersweet or outright a downer versus happily ever after?
38. Have the protagonists actually made the situation worse for their actions?
39. Do the protagonists ever bemoan the pointlessness of it or engage in an existentialist crisis without a clear answer?
40. Do any of the "bad guys" escape without suffering any karmic punishment for their actions or even end up better off than they started?
There you go. There's plenty of other ways to show whether a work is grimdark or not but this should give you a rough estimate even if you decide to drink with Coca Cola or Pepsi versus any sort of alcoholic spirit.