Since the song is both known by Traveling Matt and Pa Gorg, it's got to be an old, old song that was passed from one culture to the next with no known origin.
Junior is a baby when he remembers Pa singing it to him. Junior's what...473 years old? Gorgs aren't exactly the brightest crayons in the box and they may just be repeating words they heard from somewhere else without thinking about what they mean.
Think about nursery rhymes. They're nonsensical and often have a lot of weird stuff in them that kids wouldn't know, yet they recite. Curds and whey, kings horses assisting in reconstructing an egg, two kids getting seriously injured when fetching water, all kinds of things. Pa may not know what he's talking about (this isn't a surprise, Pa's pretty ignorant anyway).
If you really want to be imaginative, a Gorg long ago may have found ice cream mistakenly placed in their world by a Silly Creature. (Hey, Gorg artifacts have found their way into our world--'Ring Around the Rock' and 'The Gorg Who Would Be King' have great examples). Ice cream has been around for ages. The cones came later, but it's still a possible idea.
How did Matt know the song? He might've heard Junior singing it as he was tending the garden and it got passed along to the Fraggle world, just like songs and stories are passed on from one culture to the next in our world. Matt was young when he discovered the Gorg's garden, and the flashback scene shows Matt as an older Fraggle, so that's possible.
Another mind bender...Gorgs know what zebras are, although they call them 'gazebos'. According to Pa, "a gazebo is a big, smelly animal with black and white stripes. Sure, they run fast, but who needs 'em?"