It's because the word immediately preceding creionului (in other words, the declined word) is not articulated. When this happens, you need to insert a word to indicate the possessive, conforming the gender of the noun:

feminine = a masculine = al feminine plural = ale masculine plural = ai

Since "guma" is feminine, "a" is used. Another example:

Gumele de șters ale creioanelor sunt mici. (The pencils' erasers are small)

Because that means that the pencil's eraser is small, it's a possessive prefix.