|The following information is from the Adventure Time comic book series.|
It may not be part of the same canon or continuity as the animated series.
The Gum child (not to be confused with the Gum Hive) is a creature created by Princess Bubblegum several years ago. She wanted someone to succeed her as the leader of the Candy Kingdom but didn't actually want to raise a child. After her attempt to inject her DNA into other Candy People failed, she used her DNA to create a blob of bubblegum and gave it life with a Candy Heart. Unfortunately, the Gum that was created was too sweet and she attempted to spread out and hug everyone all the time. Princess Bubblegum was able to relocate her to a nearly bottomless hole in the ground marked "Gum Kingdom". During its stay there it somehow learned how to control minds by placing a piece of herself onto people's brains. After the Gum's failure, Princess Bubblegum repeated the experiment, but used much less sweetness, resulting in Lemongrab being created.
The Gum would later return to the Candy Kingdom, spreading endlessly across the countryside, hugging the entire area for miles and using her mind control power make everyone love her. She would have covered all of Ooo, however Princess Bubblegum and Marceline noticed that Lemongrab's Earldom wasn't affected by the gum. Realizing that sweet and sour are natural enemies, they cover Jake in Lemongrab's lemon juice and has him stretch over the Gum, freeing everyone and causing the Gum to recede back to her kingdom until she could learn boundaries.
She can stretch her body endlessly and can control minds by inserting herself into a person's head and placing a piece of herself on the person's brain however she gets weakened by lemon juice.