Different Villain/Antagonist interpretation:

I don't see Tom as a classic antagonist. He does not strive against the protagonist/Gatsby except to defend against Gatsby's aggression, resisting his attempts to steal his wife and destroy his family by having him investigated and exposing Gatsby's lies and criminal activities. Gatsby is the aggressor. Tom's not a nice guy, but he vows to reform and treat Daisy better and he takes action to protect his family from the deranged gun-toting George Wilson. Tom's action results in Gatsby's death, but this was defensive action, not aggression.

And two wrongs don't make a right. Tom's character flaws don't justify Gatsby's aggression.

"...from the standpoint of a character whose actions and decisions overtly prevent the protagonist from achieving his goal, I wonder if that doesn't best describe Daisy Buchanan.

What Gatsby wants most of all is entrance into Daisy's world and she is the key to that splendor."

Nick's romantic bias toward Gatsby must be discounted to get at the truth about Gatsby's feelings for Daisy. Gatsby's actions speak louder than Nick's speculation, and those actions indicate that Gatsby's goal is wealth. Look at his trophies--the castle, the cars, his excessive wardrobe, and the parties. Divorce yourself from Nick's speculative imagination and ponder what objective and reliable evidence indicates that Daisy is not just another trophy. Did she not have even one memorable keepsake from him? No. A cache of precious love letters? No. Not even a friendship ring? No.

The absence of the slightest corroboration of Nick's exaggerated view of Gatsby's love for Daisy is either a gaping weakness in the novel or an authorial clue from Fitzgerald that readers should question Nick's reliability.

The light at the end of Daisy's dock that Gatsby symbolically reaches toward is green, like money.
"Her voice is full of money," Gatsby said (page 120.) ---that was the inexhaustible charm that rose and fell in it, the jingle of it, the cymbals' song of it.... High in a white palace the king's daughter, the golden girl…

For all we know, since we aren't privy to Gatsby's thoughts, Daisy could merely be a target for him to accumulate more wealth by exploiting her romantic attraction to sell her or Tom or their rich friends worthless illicit bonds.

But readers can't objectively evaluate the quality of Gatsby's feelings because of the limitation of first-person point of view that restricts us to Nick's sometimes hyperbolic interpretation.

Based on his actions and the actions of others toward him, Gatsby's corrupt character is his worst enemy, for it is his criminality that leads Daisy to reject him when Tom unmasks him at the hotel.

When Tom bluntly exposed Gatsby's illicit sources of wealth, Daisy was awakened to the sobering prospect that her former lover's corruption will inevitably lead to prison or a life on the run or death or (gasp!) shunning by her social peers, perhaps even Jordan.

It is Gatsby's corruption, the "foul dust that floated in the wake of his dreams" that Daisy rejects, as would any sensible woman.

Gatsby's antagonist is his own corrupt character.