The keshetra of the Anandaful , Beautiful Golden Womb - Golden Egg known as Hiranyagarbha the seed of all is located where ? Is it Beyond Nirguna Brahman or is it within Sarguna Brahman?
Hiranyagarbha literally means the Golden Womb or by extension - One born from the Golden Womb - which refers to Brahma's birth from the golden egg that became the Universe. The famous Hiranyagarbha Sukta of the Rigveda states:
IN the beginning rose Hiranyagarbha, born Only Lord of all created beings. He fixed and holdeth up this earth and heaven. What God shall we adore with our oblation?
Giver of vital breath, of power and vigour, he whose commandments all the Gods acknowledge - The Lord of death, whose shade is life immortal. What God shall we adore with our oblation?
Who by his grandeur hath become Sole Ruler of all the moving world that breathes and slumbers; He who is Lord of men and Lord of cattle. What God shall we adore with our oblation?
His, through his might, are these snow-covered mountains, and men call sea and Rasā his possession:His arms are these, his are these heavenly regions. What God shall we adore with our oblation?
By him the heavens are strong and earth is steadfast, by him light's realm and sky-vault are supported:By him the regions in mid-air were measured. What God shall we adore with our oblation?
To him, supported by his help, two armies embattled look while trembling in their spirit, When over them the risen Sun is shining. What God shall we adore with our oblation?
What time the mighty waters came, containing the universal germ (Hiranya-garbha), producing Agni, Thence sprang the Gods’ one spirit into being. What God shall we adore with our oblation?
He in his might surveyed the floods containing productive force and generating Worship. He is the God of gods, and none beside him. What God shall we adore with our oblation?
Never may he harm us who is earth's Begetter, nor he whose laws are sure, the heavens' Creator, He who brought forth the great and lucid waters. What God shall we adore with our oblation?
The above verses sort of confirm what we know about Hirayagarbha in the form of Brahma - he was born from the waters, he created the earth, mid-air and heavens, he is also the ruler of living beings as well as non-living structures like mountains and seas. The Devi Bhagwat Puran also reaffirms Brahma as Hiranyagarbha:
The powerful Svâyambhuva Manu, the First, the husband of S'atarûpâ, the prosperous and the Ruler of all the Manvantaras worshipped the sinless Prajâpati Brahmâ, his Father with due devotion and satisfied Him when the Grandsire of the Lokas, the Hiranyagarbha spoke to his son:-- The excellent worship of the Devî should be done by you. By Her Grace, O Son, your work of creating worlds will be successful. Thus spoken by Brahmâ, the Bibhu Svâyambhuva Manu, the Virât incarnate, worshipped the World Mother with great austerities. And with his concentrated devotion, he satisfied the Devî Deves'î and began to chant hymns to Her, the First-born, the Mâyâ, the S'akti of all, and the Cause of all causes.
Therefore Hiranyagarbha is an epithet of Brahma who resides in the Brahmaloka.
In Hindu scriptures Hiranyagarbha is actually associated with one the Trinities viz: Lord BrahmA.
Now, Lord BrahmA has many GAyatri Mantras dedicated to him. One such Mantra is the following:
Om vedAtmanAya vidmahe hiranyagarbhAya dhimahi tanno brahmA prachodayAt ||
So, here Lord BrahmA is explicitly identified with Hiranyagarbha.
Thus, the answer to your current question is "Brahmaloka", which is the abode of Lord BrahmA.
No, Hinranyagarbha is not beyond Nirguna Brahman. At the onset of creation Brahman manifests himself as Hiranyagarbha and pervades the creation. Other names for Hiranhyagarbha are MahAnAtmA, SutrAtmA etc.