Gabelspaghetti, or Gabelnudeln, are a type of egg noodle created and sold by German pasta brands. They are - one of several misnomers regarding this product - not Spaghetti and bear very little resemblance to them. The simple reason they are named like this might be that Spaghetti are perhaps the most popular type of pasta.
However, the one thing similar to Spaghetti with these noodles is the thinness - a Gabelspaghetti is about as thick as the famous Italian pasta type. Now imagine a Spaghetti that, despite being thin already, is hollow inside, like a Maccaroni. Now cut off roughly one inch and put your tiny hollow Spaghetti around your little finger like a ring. Take it off but let the noodle keep this shape, and then cut it in half. That's how a Gabelspaghetti looks.
Gabelspaghetti are typically made of 75-80% wheat and 20-25% eggs, although there are egg-free variants, and ones with salt. They are only available as dry pasta and most commonly retail in 400g and 500g packs. It's a very versatile noodle: You can simply serve them with sauce, but also in soups and stews, in pasta salads (quite common in Germany), or on their own as a side.
The popularity of Gabelspaghetti is perhaps closely related to their suitability for kids. The noodles are able to draw a lot of sauce like Maccaroni, but small children are a lot less likely to make a mess with the sauce: Gabelspaghetti are easy to handle because - another major misnomer regarding this product - you can eat them with a spoon, despite "Gabel" meaning "fork".
No interesting origin story for this noodle exists. It's not a main part of any traditional cuisine or regional signature dish. It's a relatively recent "invention", and its popularity is mostly confined to German speaking regions, from what I know. All this results in very little being written about Gabelspaghetti, perhaps making this little writeup the most exhaustive source of information on the topic available.