I think that the pink gooey stuff that was In the episode Simon and Marcy was a toxic thing that turned whoever touches it into bubblegum and that's how princess bubblegum was 'made'.

Pink goo
I think that the pink gooey stuff that was In the episode Simon and Marcy was a toxic thing that turned whoever touches it into bubblegum and that's how princess bubblegum was 'made'.