What class of Devil Fruit does the Gomu Gomu no Mi come under?
From the wiki.


  The Gomu Gomu no Mi is a Paramecia-type Devil Fruit that enables the user's body to stretch like rubber, making the user a Rubber Human.


You further ask..


  It's very close to a Paramecia but still doesn't have a logical explanation to be of Paramecia type!


Again from the wiki.


  Paramecia is one of the three Devil Fruit types. These fruits give the users a power that can affect their body, manipulate the environment, or produce substances. In general, Paramecia Devil Fruits are fruits that give their users powers other than transforming into elements, like Logias, or transforming into animals, like Zoans.