How would you describe an action that is grossly wicked or reprehensible?

The term that best describes an action that is grossly wicked or reprehensible is "heinous." This word specifically refers to acts that are shockingly evil or wicked, and it is often used in legal contexts to describe particularly severe crimes. The connotation of heinous includes an inherent moral outrage, capturing the intense disapproval associated with such actions.

In contrast, the other words do not align with the concept of wickedness or reprehensibility. "Gullible" describes someone who is easily deceived or tricked, lacking the connection to moral wrongdoing. "Grovel" refers to the act of showing excessive humility or subservience, which does not imply any inherent evil. "Guile" means clever or cunning intelligence, often involving deceit, but it does not specifically denote an act of wrongdoing to the extent suggested by "heinous." Therefore, "heinous" is the most appropriate choice for characterizing grossly wicked actions.