Where do we find the Ganapati Atharvasheersh in scriptures? What is the meaning of the word 'Atharvsheersh'. Who is the writer of it (Rishi Ganak or Atharvan)?
The Ganapati Atharvashirsha Upanishad, AKA the Ganesha Upanishad or Ganapati Upanishad, is a text which claims to be an Upanishad associated with the Atharvana Veda; here is what it says about itself in verses 16-18 (see pages 5-7 of this PDF file):
He who studies this Atharva text moves towards Brahman.... This Atharva text should not be given to those not pupils.... This is an Atharva saying: "He who moves towards Brahmavidya is never afraid."
The last sentence is where it gets the name Atharvashirsha, or "head of Atharva", because some interpret the line not as an introduction of a quote as this translation renders it, but rather as a statement that the text as a whole was said by the sage Atharva, i.e. that it came right from the head of Atharva.
Now having said that, it should be noted that the Ganapati Atharvashirsha Upanishad is widely considered to be a later interpolation, not a genuine Upanishad of the Atharvana Veda. This is for multiple reasons. First of all, references to Ganesha in ancient Hindu scripture are few and far between, as Ganesha rose to popularity much later. But more importantly, it's not listed in the Muktika, the canonical list of Upanishads, which means that it's presumably less than 400 years old. Let me explain why.
Hindu scripture was passed down via oral tradition, and there were some good oral tradition processes and some bad ones. An example of an incredibly rigorous oral tradition process is the one used to preserve the Samhitas of the Vedas, the core part of the Vedas whose verses were heard directly from the gods. Because these verses were of divine origin, people took great pains to preserve every last syllable exactly, such as parallel disciplic successions and independent cross-checks. That is how scholars are so sure that, e.g. the Rig Veda Samhita as we have it today is almost the exact same document that the sage Vyasa compiled.
But other texts weren't so rigorously prserved. In particular, the Muktika Upanishad, a text listing the names of the Upanishads of each the Vedas, was passed down with so little rigor that anyone in the disciplic succession could freely add in titles of Upanishads as he saw fit. This wasn't done maliciously; it was just students innocently thinking that the text they had received was incomplete, since they were aware of supposed Upanishads which were not mentioned. This is how Upanishads that are now widely considered to be spurious, like the Kali Santarana Upanishad as I discuss in this question, were added onto the Muktika canon. The text of the Muktika only started becoming stable once it was committed to writing, but that was only in the seventeenth century. So the Muktika is basically a list of all the documents that people even remotely considered to be an Upanishad up until the seventeenth century.
Yet the Ganapati Atharvashirsha Upanishad is not even listed in the Muktika Upanishad, which you can read here. That means that none of the students in the disciplic successions, going all the way up to a few hundred years ago, ever added it to the text, which means that they presumably hadn't heard of it. So that implies that origin of the text, or at least the time when it started being considered an Upanishad, was newer than when the Muktika Upanishad was committed to writing.