The Blockheads are the series' main antagonists and are humanoid, red-colored figures with block-shaped heads, who wreak mischief and havoc at all time. There are 26 blockheads, each having a certain letter on both sides of their heads, though the two most featured blockheads are Blockhead G and Blockhead J, who are Gumby's nemeses. Blockhead G is most likely the dominant one of the duo, while Blockhead J usually gets slapped more often.
The Blockheads never speak, communicating purely through gestures or pantomiming. Their reasons behind terrorizing Gumby and those around him are never elaborated on, suggesting they are simply malevolent in nature. The 2017 comic revival, however, reveals they are evil because they are a minority group oppressed by the clay people simply due to the shape of their heads, and therefore, desire revenge.
- The Blockheads were inspired by the Katzenjammer Kids, who were always getting into scrapes and causing discomfort to others.
- Four additional blockheads with the letters A, X, Y, & Z appeared in "The Racing Game" as a pit crew. Blockhead X and Blockhead Z would later appear in "Band Contest" as members of The Block Rockers with Blockhead G & Blockhead J. Two more additional blockheads with the letters B, & C appear in "The Gumby League" as part of the Blockheads baseball team.
- Even though the Blockheads never speak, they can be heard laughing in Gumby's Adventures, usually at the expense of Gumby & his pals.
- Blockhead J's letter in the 1960s series is mostly seen reversed in some episodes, while Blockhead G's letter remains the same. Whether this was overlooked, an error, or intentional, is unknown.
- Curiously, Blockhead J was green in the ending music video, "Take Me Away," in Gumby the Movie.